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Currently a call to |GestureRecognizerImpl::TransferEventsTo()| can lead
to an incorrect state for touch events. Specifically, the event ACKS are
getting routed to a different reference of GestureProvider from the one
where the original events were routed to.

This patch stores a mapping of every event that was dispatched along
with its corresponding GestureProvider. This allows the event ACKS to be
routed correctly despite a call made to |TransferEventsTo()|.

BUG=698843
COMPONENT=Gesture Recognizer, Touch Gestures, Drag and Drop

Review-Url: https://codereview.chromium.org/2789203006
Cr-Commit-Position: refs/heads/master@{#462925}