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1. Add OnMenuClosed to notify observers when context menu is dismissed.
2. Add OnContextMenuShown to notify observers when context menu is
shown.

Bug: 1020004
Change-Id: I61f8c44edd909f3ba5e0566cb60cb79b59433091
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2055357Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Commit-Queue: Xiao Yang <yanxiao@google.com>
Cr-Commit-Position: refs/heads/master@{#743369}