Here is what I *think* needs to be done:

- Replace the contents of google_update/google_update_idl.idl with the server-side definitions in the omaha3_idl.idl file, excluding the GoogleComProxyMachineClass and GoogleComProxyUserClass interfaces.
- Update the generated files in third_party/win_build_output/midl/google_update/{x64,x86}

If the above are the only things that need to be updated, then I can take a first stab at this.

Greg, do you know if an update like this is safe? Just asking in case you know of anything seemingly unrelated that could break.

I didn't even know about this win_build_output until now. TIL...

This plan seems fine to me. Please test manually by making an "is_chrome_branded = true" build, installing it on a machine that already has GU installed, and checking that a manual update check via chrome://help works.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/21489fa99e78a80547d9e537f7cda88cf604e328

commit 21489fa99e78a80547d9e537f7cda88cf604e328
Author: Ali Tofigh <alito@chromium.org>
Date: Tue Feb 27 16:41:24 2018

Update Google Update COM interface definitions in google_update_idl.idl.

This update is required for work that is underway related to management
of Chrome browsers for enterprise users.

The google_update_idl.idl file was originally provided by the Google
Update team. It has been modified as described in the README.chromium
file in this CL.

The change was tested with Chrome-branded builds with target_cpu set to
"x86" and "x64". I followed these steps to make sure that this change
does not break Chrome updates.

1. Manully set the version of Chrome in src/chrome/VERSION to a version
   prior to that which is currently served on the Dev channel.
2. Compile with is_chrome_branded=true and target_cpu set to one of
   "x86" or "x64".
3. Install the build as a Dev-channel Chrome using setup.exe on a 64-bit
   Win10 machine.
4. Navigate to chrome://help and make sure that the update process works
   as expected.

Bug:  796270 
Change-Id: I6962b8874a7e825d41834860de0eb0fe92f5a6e3
Reviewed-on: https://chromium-review.googlesource.com/924474
Commit-Queue: Ali Tofigh <alito@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539457}
[modify] https://crrev.com/21489fa99e78a80547d9e537f7cda88cf604e328/google_update/README.chromium
[modify] https://crrev.com/21489fa99e78a80547d9e537f7cda88cf604e328/google_update/google_update_idl.idl
[modify] https://crrev.com/21489fa99e78a80547d9e537f7cda88cf604e328/third_party/win_build_output/midl/google_update/x64/google_update_idl.h
[modify] https://crrev.com/21489fa99e78a80547d9e537f7cda88cf604e328/third_party/win_build_output/midl/google_update/x64/google_update_idl.tlb
[modify] https://crrev.com/21489fa99e78a80547d9e537f7cda88cf604e328/third_party/win_build_output/midl/google_update/x64/google_update_idl_i.c
[modify] https://crrev.com/21489fa99e78a80547d9e537f7cda88cf604e328/third_party/win_build_output/midl/google_update/x64/google_update_idl_p.c
[modify] https://crrev.com/21489fa99e78a80547d9e537f7cda88cf604e328/third_party/win_build_output/midl/google_update/x86/google_update_idl.h
[modify] https://crrev.com/21489fa99e78a80547d9e537f7cda88cf604e328/third_party/win_build_output/midl/google_update/x86/google_update_idl.tlb
[modify] https://crrev.com/21489fa99e78a80547d9e537f7cda88cf604e328/third_party/win_build_output/midl/google_update/x86/google_update_idl_i.c
[modify] https://crrev.com/21489fa99e78a80547d9e537f7cda88cf604e328/third_party/win_build_output/midl/google_update/x86/google_update_idl_p.c

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a8a76231abb6229e9a3e1102331b452fcdd84e77

commit a8a76231abb6229e9a3e1102331b452fcdd84e77
Author: Ali Tofigh <alito@chromium.org>
Date: Tue Feb 27 21:22:59 2018

Revert "Update Google Update COM interface definitions in google_update_idl.idl."

This reverts commit 21489fa99e78a80547d9e537f7cda88cf604e328.

Reason for revert: This broke the official builders because I did not realize that google_update_win_unittest.cc needs to be updated.

Original change's description:
> Update Google Update COM interface definitions in google_update_idl.idl.
> 
> This update is required for work that is underway related to management
> of Chrome browsers for enterprise users.
> 
> The google_update_idl.idl file was originally provided by the Google
> Update team. It has been modified as described in the README.chromium
> file in this CL.
> 
> The change was tested with Chrome-branded builds with target_cpu set to
> "x86" and "x64". I followed these steps to make sure that this change
> does not break Chrome updates.
> 
> 1. Manully set the version of Chrome in src/chrome/VERSION to a version
>    prior to that which is currently served on the Dev channel.
> 2. Compile with is_chrome_branded=true and target_cpu set to one of
>    "x86" or "x64".
> 3. Install the build as a Dev-channel Chrome using setup.exe on a 64-bit
>    Win10 machine.
> 4. Navigate to chrome://help and make sure that the update process works
>    as expected.
> 
> Bug:  796270 
> Change-Id: I6962b8874a7e825d41834860de0eb0fe92f5a6e3
> Reviewed-on: https://chromium-review.googlesource.com/924474
> Commit-Queue: Ali Tofigh <alito@chromium.org>
> Reviewed-by: Will Harris <wfh@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Reviewed-by: Greg Thompson <grt@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#539457}

TBR=thakis@chromium.org,wfh@chromium.org,grt@chromium.org,alito@chromium.org

Change-Id: Idac53b550e713d95f131cb0fe8ea189f88d89a26
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug:  796270 
Reviewed-on: https://chromium-review.googlesource.com/939652
Reviewed-by: Ali Tofigh <alito@chromium.org>
Commit-Queue: Ali Tofigh <alito@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539549}
[modify] https://crrev.com/a8a76231abb6229e9a3e1102331b452fcdd84e77/google_update/README.chromium
[modify] https://crrev.com/a8a76231abb6229e9a3e1102331b452fcdd84e77/google_update/google_update_idl.idl
[modify] https://crrev.com/a8a76231abb6229e9a3e1102331b452fcdd84e77/third_party/win_build_output/midl/google_update/x64/google_update_idl.h
[modify] https://crrev.com/a8a76231abb6229e9a3e1102331b452fcdd84e77/third_party/win_build_output/midl/google_update/x64/google_update_idl.tlb
[modify] https://crrev.com/a8a76231abb6229e9a3e1102331b452fcdd84e77/third_party/win_build_output/midl/google_update/x64/google_update_idl_i.c
[modify] https://crrev.com/a8a76231abb6229e9a3e1102331b452fcdd84e77/third_party/win_build_output/midl/google_update/x64/google_update_idl_p.c
[modify] https://crrev.com/a8a76231abb6229e9a3e1102331b452fcdd84e77/third_party/win_build_output/midl/google_update/x86/google_update_idl.h
[modify] https://crrev.com/a8a76231abb6229e9a3e1102331b452fcdd84e77/third_party/win_build_output/midl/google_update/x86/google_update_idl.tlb
[modify] https://crrev.com/a8a76231abb6229e9a3e1102331b452fcdd84e77/third_party/win_build_output/midl/google_update/x86/google_update_idl_i.c
[modify] https://crrev.com/a8a76231abb6229e9a3e1102331b452fcdd84e77/third_party/win_build_output/midl/google_update/x86/google_update_idl_p.c

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2376c7e78e63d894ee2652bcc88d4c54ac96229f

commit 2376c7e78e63d894ee2652bcc88d4c54ac96229f
Author: Ali Tofigh <alito@chromium.org>
Date: Thu Mar 01 00:29:13 2018

Reland "Update Google Update COM interface definitions in google_update_idl.idl."

This is a reland of 21489fa99e78a80547d9e537f7cda88cf604e328.

Original change's description:
> Update Google Update COM interface definitions in google_update_idl.idl.
>
> This update is required for work that is underway related to management
> of Chrome browsers for enterprise users.
>
> The google_update_idl.idl file was originally provided by the Google
> Update team. It has been modified as described in the README.chromium
> file in this CL.
>
> The change was tested with Chrome-branded builds with target_cpu set to
> "x86" and "x64". I followed these steps to make sure that this change
> does not break Chrome updates.
>
> 1. Manully set the version of Chrome in src/chrome/VERSION to a version
>    prior to that which is currently served on the Dev channel.
> 2. Compile with is_chrome_branded=true and target_cpu set to one of
>    "x86" or "x64".
> 3. Install the build as a Dev-channel Chrome using setup.exe on a 64-bit
>    Win10 machine.
> 4. Navigate to chrome://help and make sure that the update process works
>    as expected.
>
> Bug:  796270 
> Change-Id: I6962b8874a7e825d41834860de0eb0fe92f5a6e3
> Reviewed-on: https://chromium-review.googlesource.com/924474
> Commit-Queue: Ali Tofigh <alito@chromium.org>
> Reviewed-by: Will Harris <wfh@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Reviewed-by: Greg Thompson <grt@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#539457}

Bug:  796270 
Change-Id: I13c1fd5446ca8278ba43f57181fa5994e89608a8
Reviewed-on: https://chromium-review.googlesource.com/939656
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Ali Tofigh <alito@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539956}
[modify] https://crrev.com/2376c7e78e63d894ee2652bcc88d4c54ac96229f/chrome/browser/google/google_update_win_unittest.cc
[modify] https://crrev.com/2376c7e78e63d894ee2652bcc88d4c54ac96229f/google_update/README.chromium
[modify] https://crrev.com/2376c7e78e63d894ee2652bcc88d4c54ac96229f/google_update/google_update_idl.idl
[modify] https://crrev.com/2376c7e78e63d894ee2652bcc88d4c54ac96229f/third_party/win_build_output/midl/google_update/x64/google_update_idl.h
[modify] https://crrev.com/2376c7e78e63d894ee2652bcc88d4c54ac96229f/third_party/win_build_output/midl/google_update/x64/google_update_idl.tlb
[modify] https://crrev.com/2376c7e78e63d894ee2652bcc88d4c54ac96229f/third_party/win_build_output/midl/google_update/x64/google_update_idl_i.c
[modify] https://crrev.com/2376c7e78e63d894ee2652bcc88d4c54ac96229f/third_party/win_build_output/midl/google_update/x64/google_update_idl_p.c
[modify] https://crrev.com/2376c7e78e63d894ee2652bcc88d4c54ac96229f/third_party/win_build_output/midl/google_update/x86/google_update_idl.h
[modify] https://crrev.com/2376c7e78e63d894ee2652bcc88d4c54ac96229f/third_party/win_build_output/midl/google_update/x86/google_update_idl.tlb
[modify] https://crrev.com/2376c7e78e63d894ee2652bcc88d4c54ac96229f/third_party/win_build_output/midl/google_update/x86/google_update_idl_i.c
[modify] https://crrev.com/2376c7e78e63d894ee2652bcc88d4c54ac96229f/third_party/win_build_output/midl/google_update/x86/google_update_idl_p.c

The reland CL seems to have stuck this time. The CL went into Canary version 66.0.3358.0, which I have tested: the update check from chrome://help seems to be working.

I'll hold off on marking this fixed until I've tested a couple more Canary versions in the coming days.

Updates on Canary seem to be working fine; I've gotten more than two updates now. Marking this as fixed.