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Currently when we tab through the action bar we cannot focus on the
sharesheet button. This CL fixes this issue and also add a test for this
bug.

The issue with the original implementation is because we disable the
command when we are waiting for feedback from the C++ api, however when
we tab through the action bar, the canExecute() is called and we cannot
focus on the sharesheet button while tabing because the command is still
disabled at the time. Changing the code to keep the command enable if
the event target is action bar. This should be safe because in this
event, there should be no change to the selected entry so if the button
is visible at the time, it should be safe to be enabled.

BUG=1136334, 1097623

Change-Id: I285bb7fe71322c05ae660fb02917126e7491b17a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2460854Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Maggie Cai <mxcai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816063}