For UI leads' benefit: the current behavior is problematic for users who aren't actually using Dev SxS with another install of Chrome (for example, me).  Since we don't offer a non-SxS installer (that I could find), it becomes much more difficult to use Dev and Beta as "daily driver" browsers.

Dev and Beta should not prompt users to make them default (e.g. via an infobar) if another install of Chrome is default, but if it isn't, prompting is fine, and in either case, allowing the user to register them as default from Chrome's settings is important, since especially in Win 10 doing this correctly from the control panel is harder.  I think this is what grt is proposing.

Thanks for chiming in, Peter. Yes, that's pretty much what I was thinking of. I've already started looking at the technical aspects of implementing this. I'll send out a doc with some points to consider just to be sure everyone's aware of the possibilities.

Design doc: https://docs.google.com/a/chromium.org/document/d/1y5DDjJoKyacbaDEBzhs_DOa6AJ8Cs4k2uKEoiv20BZ4/edit?usp=sharing

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/75b1c8e57ff391eed3fb61fe7058a536368de545

commit 75b1c8e57ff391eed3fb61fe7058a536368de545
Author: Greg Thompson <grt@chromium.org>
Date: Wed Aug 16 13:36:28 2017

Enable default browser UX in side-by side dev and beta installs.

- ShellUtil::GetChromeDefaultState and
  GetChromeDefaultProtocolClientState now tell the caller if a different
  side-by-side install is the default handler.
- chrome://settings now shows the default browser UX for side-by-side
  channels that support setting as default (i.e., dev and beta).
- The Win10 welcome page, Win8 welcome dialog, and default browser
  infobar are all suppressed when another install of Chrome is default.

BUG= 752007 

Change-Id: I539325fa38a3de247209d0c4c17ee4ff58cfc0ef
Reviewed-on: https://chromium-review.googlesource.com/611801
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Patrick Monette <pmonette@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#494759}
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/browser/custom_handlers/protocol_handler_registry.cc
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/browser/external_protocol/external_protocol_handler_unittest.cc
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/browser/shell_integration.cc
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/browser/shell_integration.h
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/browser/shell_integration_win.cc
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/browser/ui/apps/chrome_app_delegate.cc
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/browser/ui/startup/default_browser_prompt.cc
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/browser/ui/startup/default_browser_prompt_win.cc
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/browser/ui/startup/startup_browser_creator_impl.cc
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/browser/ui/startup/startup_tab_provider.cc
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/browser/ui/webui/set_as_default_browser_ui_win.cc
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/browser/ui/webui/welcome_win10_handler.cc
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/install_static/google_chrome_install_modes.cc
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/installer/util/beacons.cc
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/installer/util/shell_util.cc
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/chrome/installer/util/shell_util.h
[modify] https://crrev.com/75b1c8e57ff391eed3fb61fe7058a536368de545/tools/metrics/histograms/enums.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/260604e120effcb3f6c1c4706486172b40844899

commit 260604e120effcb3f6c1c4706486172b40844899
Author: Greg Thompson <grt@chromium.org>
Date: Wed Aug 23 06:26:42 2017

Fix browser registration for per-user dev and beta installs on Win7.

This change also makes the handling of the make_chrome_default and
make_chrome_default_for_user master preferences options conditional on a
brand's support for in-product default browser handling.

BUG= 752007 
R=pmonette@chromium.org

Change-Id: I1eb6d4344aa845943e726b69cf2319e6cf056cb9
Reviewed-on: https://chromium-review.googlesource.com/625896
Reviewed-by: Patrick Monette <pmonette@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496600}
[modify] https://crrev.com/260604e120effcb3f6c1c4706486172b40844899/chrome/installer/setup/install.cc
[modify] https://crrev.com/260604e120effcb3f6c1c4706486172b40844899/chrome/installer/setup/setup_main.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/deea6d75b66252702526a1eda087aec15fa9672b

commit deea6d75b66252702526a1eda087aec15fa9672b
Author: Greg Thompson <grt@chromium.org>
Date: Tue Aug 29 19:32:32 2017

Fix the check for other Chrome browsers on Win 7.

While the probe was checking for other install modes being default, it
wasn't doing so across the per-user/per-machine boundary. As a result,
per-user Chrome Dev (for example) was not noticing that per-machine
stable Chrome was default. Now it does.

BUG= 752007 
R=pmonette@chromium.org

Change-Id: Idef3bcd8172f304bf075ce5c5b44a5cda3bf7588
Reviewed-on: https://chromium-review.googlesource.com/637996
Reviewed-by: Patrick Monette <pmonette@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#498199}
[modify] https://crrev.com/deea6d75b66252702526a1eda087aec15fa9672b/chrome/installer/util/shell_util.cc

Fixed in 62.0.3200.0.