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This CL:
 - Hides the Extensions icon in web app windows unless the Extensions
   menu is opened.
 - Adds an "Extensions" menu item to the web app three dot menu to
   open the Extensions menu.

This change is behind the DesktopPWAsElideExtensionsMenu flag.

Minor change in behaviour to browser window extension menu
(regardless of flag state): When the last extension is removed the
container will wait for its layout animation to complete before hiding
itself.

Before:
https://bugs.chromium.org/p/chromium/issues/attachment?aid=480233&signed_aid=3saz6ZsTFVwVy1W5EoAbXQ==&inline=1

After:
https://bugs.chromium.org/p/chromium/issues/attachment?aid=480997&signed_aid=A3ERk4v_G7ywyM9S3Wx10A==&inline=1

Bug: 1155421
Change-Id: I80d5b382610895021c2cdf861d253edbdabe8630
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2573897
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838031}