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The current behavior makes TransferEventsTo() before the window
move starts, but don't care at its end. This causes errors like
crbug.com/900363. But we can't simply transfer back to the original
window unconditionally since someone (like TabDragController) may
also want to continue dragging on a window other than the original
source window.

This CL introduces a new scoped class to control this; it makes
TransferEventsTo first, and then invokes TransferEventsTo back
again at the end, but it skips invoking the second transfer if
someone else also transfers the touch events on the same window.

BUG=900363
TEST=the new test case

Change-Id: I81792872a13f0e4bb2c88b526c92cf689d656b71
Reviewed-on: https://chromium-review.googlesource.com/c/1310501
Commit-Queue: Jun Mukai <mukai@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604473}