GitLab

UX requests we disable context menus on the back button and overflow
buttons in the shelf.

Removed tests for these cases and added new ones for not showing
the menus.

Bug: 875334
Change-Id: I45c320a148e8f7ad39abd0e8181e59e3c4b663fe
Reviewed-on: https://chromium-review.googlesource.com/1180073Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584219}