New synthetic gestures implementation.

New SyntheticGesture and SyntheticGestureController classes.
The gesture controller handles a queue of gestures that get processed one after another. It pings the gestures to forward input events on a fixed interval.

Added SyntheticGestureTarget interface to link up the gesture controller with the platform.

BUG=297980

Review URL: https://codereview.chromium.org/27232002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231008 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231008