The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5058fb8c58396c30646b7029a4ef546602858c14

commit 5058fb8c58396c30646b7029a4ef546602858c14
Author: dpapad <dpapad@chromium.org>
Date: Tue May 29 18:48:24 2018

Delete base::Feature kMaterialDesignExtensions

The feature flag is no longer necessary, since the old UI has been deleted
already, and the new UI is always served regardless of the feature's value.

Bug:  846897 
Change-Id: I782fa5e931d8da959119b73a77538a87ff425c80
Reviewed-on: https://chromium-review.googlesource.com/1073951
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562526}
[modify] https://crrev.com/5058fb8c58396c30646b7029a4ef546602858c14/chrome/browser/about_flags.cc
[modify] https://crrev.com/5058fb8c58396c30646b7029a4ef546602858c14/chrome/browser/flag_descriptions.cc
[modify] https://crrev.com/5058fb8c58396c30646b7029a4ef546602858c14/chrome/browser/flag_descriptions.h
[modify] https://crrev.com/5058fb8c58396c30646b7029a4ef546602858c14/chrome/common/chrome_features.cc
[modify] https://crrev.com/5058fb8c58396c30646b7029a4ef546602858c14/chrome/common/chrome_features.h
[modify] https://crrev.com/5058fb8c58396c30646b7029a4ef546602858c14/chrome/test/data/webui/extensions/a11y/extensions_a11y_test.js
[modify] https://crrev.com/5058fb8c58396c30646b7029a4ef546602858c14/chrome/test/data/webui/extensions/cr_extensions_browsertest.js