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Currently, destroying a ui::TouchEvent removes its tracking id from the slot mapping. Because MenuEventDispatcher::DispatchEvent was creating
(and destroying) a touch event, the tracking id was being
removed too early.

This patch refcounts the tracking ids, so the tracking id won't be removed until it should be.

BUG=394437

Review URL: https://codereview.chromium.org/400213002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285580 0039d316-1c4b-4281-b951-d872f2087c98