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Click with Command on Mac, or click with control on other platforms, on
a bookmark folder should open all the bookmarks in new background tabs.
This CL adds such action.

In order to test such action, this CL also modified ui controls across
platforms to be able to handle modifier keys for mouse events. The test
added would work across platforms.

BUG=170072,841781

Change-Id: Idbe1160d7f13f8cec003f086d948252b612276ed
Reviewed-on: https://chromium-review.googlesource.com/1068253Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564258}