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When a client is notified of events it's told if the event matches a
pointer observer. If the event does match a pointer observer, a single
event is sent to the client. This implies I need a post event
processing hook to determine if the client was notified already and
not send if appropriate. This is why I added
WindowEventDispatcherObserver as a way to deal with this. I'll be more
obvious in a following patch.

BUG=837692
TEST=covered by test

Change-Id: Ifb3e332466b3e6a6ee09c625020d6dbf9ed1a54e
Reviewed-on: https://chromium-review.googlesource.com/1054413
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557797}