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This CL does 3 things:
 - Hides the Extensions menu item from web app windows when no
   Extensions are installed.
 - Fixes a bug exposed by the updated test where the Extensions puzzle
   piece would show when an Extension was installed due to the
   layout animation.
 - Addresses comments from
   https://chromium-review.googlesource.com/c/chromium/src/+/2602305

Bug: 1164829
Change-Id: I75e3fc5e8a7262316947b7322b825a10be3bb848
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2621512
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842372}