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This CL adds a context menu for suggestions. It currently only contains
a "Remove suggestion..." entry to mirror the Shift+Delete behavior,
but currently doesn't do anything.

In the future, it will be wired to a confirm bubble.

Bug: 929477
Change-Id: If12de2e04c41c4a639fd31a39a80efeac936883c
Reviewed-on: https://chromium-review.googlesource.com/c/1457234
Commit-Queue: Tommy Li <tommycli@chromium.org>
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630560}