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The context menu will only show when the content forward entry point is
enabled and will eventually be used to allow the user to enable/disable
holding space item previews in the tray.

This CL adds and shows the context menu but does not wire it up to
perform its intended action. That will be done in the next CL.

Bug: 1142572
Change-Id: Ic81bf57b84467d28757b311e73aeacf0eef401b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523905Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#825426}