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This is simpler and less error-prone, and will allow moving the app menu
observer access point to the app menu button (in a future CL).

Bug: none
Change-Id: I17eaf8d3374ae3d8bf1b078631f93c8a6c2e88f7
Reviewed-on: https://chromium-review.googlesource.com/c/1447323
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628541}