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Add callback supports to the RenderWidgetTargeter
and use the async targetting in drag and drop code
and register a callback to send drag events using
the callback. Note that since drag and drop events
aren't WebInputEvent it is not possible to use
FindTargetAndDispatch function of RenderWidgetTargeter
at this time.

Bug: 804633
Change-Id: I2ecea2b70d7ea619169685a3a7b531355ddc0d59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1596840
Commit-Queue: Navid Zolghadr <nzolghadr@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665239}