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These will be used by test code to trigger events based on dragging.  Tests
can't reliably e.g. "queue event to trigger a drag and then call a particular
callback" due to the nested, filtered message loop on Windows.  Using drag
observer methods is reliable, and clearer as well.

Bug: 923188
Change-Id: Id24b8150d04dc878dce543fe469994aba29694e4
Reviewed-on: https://chromium-review.googlesource.com/c/1483931
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634869}