CL is up: https://chromium-review.googlesource.com/c/chromium/src/+/1311353

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3116428824794f0b0eab25cba3a0ad3e85231309

commit 3116428824794f0b0eab25cba3a0ad3e85231309
Author: Pavol Marko <pmarko@chromium.org>
Date: Mon Nov 05 11:02:08 2018

Avoid duplicate names in generated ADMX schema files

If two exension managed schema policy properties (at different hierarchy
levels) have the same id, the ADMX generator should not generate the
same name attribute for the corresponding ADMX elements.
If the name is reused, the GPO editor rejects the ADMX file.
Use 'full name's instead, which are built from the names of all
hierarchy levels.

Bug:  900517 
Test: manual
Change-Id: Ief2edeb71defb15a768ea7df9e7366796bab728e
Reviewed-on: https://chromium-review.googlesource.com/c/1311353
Reviewed-by: Alexander Hendrich <hendrich@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605291}
[modify] https://crrev.com/3116428824794f0b0eab25cba3a0ad3e85231309/components/policy/tools/generate_extension_admx.py

Status: Waiting for Peter to try out the new version before marking this Fixed / continuing work on this.

Peter -- friendly ping -- did this solve the issue?
Thanks!

Marking as Fixed based on offline feedback by Peter.

Hi..
Please provide steps if this requires manual verification. 

I think we can skip this here since it's a one-off tool not used in production. Peter already has verified that the modified templates work.

Thanks ljusten@ 
Marking the issue as verified.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bdef067269abb075609305651afb7e00fc4e2722

commit bdef067269abb075609305651afb7e00fc4e2722
Author: Lutz Justen <ljusten@chromium.org>
Date: Mon Nov 19 15:19:26 2018

generate_extension_admx.py: Simplify root category name

Just use 'extension' instead of 'extension_<32-byte-hash>' as root
category name for brevity. The hash blew up the ADMX files quite a bit
since the root category is prepended to pretty much every ID.

Note that the categories are already namespaced by the hash, so the hash
is not necessary.

Bug:  900517 
Test: manual
Change-Id: I74ec7d3463486525139bf895edbea84bef0afe12
Reviewed-on: https://chromium-review.googlesource.com/c/1338081
Commit-Queue: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609297}
[modify] https://crrev.com/bdef067269abb075609305651afb7e00fc4e2722/components/policy/tools/generate_extension_admx.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/63b4d45fbd2e2db84799b8b500a23ae805511f7d

commit 63b4d45fbd2e2db84799b8b500a23ae805511f7d
Author: Lutz Justen <ljusten@chromium.org>
Date: Mon Dec 03 14:43:58 2018

generate_extension_admx.py: Fix value names

Fixes a bug where the wrong registry value names where used, so that
setting policy didn't have effect. It used a hierarchical value name
  cat1_cat2_cat3_policy
This isn't necessary since the corresponding registry key already
contains the categories:
  Software\Policies\Google\Chrome\3rdparty\extensions\<extension_id>\Policy\cat1\cat2\cat3
so that the policy is guaranteed to be unique within the key. What's
worse is that the code isn't looking for 'cat1_cat2_cat3_policy', but
just for 'policy'. Hence, the policy settings were not taken.

Bug:  900517 
Test: manual
Change-Id: I2e1b83fe2ff6b0997ab22e47c986a92dcb19559e
Reviewed-on: https://chromium-review.googlesource.com/c/1356699
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Lutz Justen <ljusten@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613080}
[modify] https://crrev.com/63b4d45fbd2e2db84799b8b500a23ae805511f7d/components/policy/tools/generate_extension_admx.py