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And this also changes ws2 to only stop cancelable events. To do otherwise
triggers a CHECK in Event.

BUG=837686
TEST=covered by tests

Change-Id: I6bc3d8b6a02eda1ecf7b7aecec6729a9ef7b1a2e
Reviewed-on: https://chromium-review.googlesource.com/1111268
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569751}