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This will be used by drag and drop tests to take action on dragging over various
views (typically, sending a mouse release event to end the drag).

Bug: 923188
Change-Id: I40a3885cf8076f9e0a5f0490ca1eda3f32907086
Reviewed-on: https://chromium-review.googlesource.com/c/1493435
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636343}