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This implements a touch dispatch model in which touchmove sending is throttled
while a scroll sequence is active and being consumed.  Such touchmove's are
marked with |cancelable = false|, indicating to any potential consumers that the
event cannot be prevented from triggering a platform gesture. Throttling limits
the touchmove sending rate during scrolling to 1 event per 200ms.

BUG=346693

Review URL: https://codereview.chromium.org/245833002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266470 0039d316-1c4b-4281-b951-d872f2087c98