GitLab

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.

Bug: 1155421
Change-Id: I474a6ced0e7dd7893317bff4d4b1d8256957d80b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2620721
Commit-Queue: Matt Giuca <mgiuca@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842249}