DragController: Clear DragState in DragEnd

DragState is a ScriptWrappable which can keep alive JavaScript upon
unified heap garbage collection resulting in a leak of the last Window
that used a drag event. Clear the state when the drag ended.

Bug: 843903, 928781
Change-Id: Id86a1a917fe772f56fb679e1d73ef3453785ce6d
Reviewed-on: https://chromium-review.googlesource.com/c/1456073
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629582}
1 file changed