Synthetic tap gesture.

Add a synthetic tap gesture implementation supporting both touch and mouse
input. It takes a position and a duration, i.e. the interval between tapping
down and lifting.
The patch also adds a corresponding JS interface to chrome.gpuBenchmarking.

Synthetic tap gestures will be used by Telemetry to benchmark sites with their own touch handlers. On these sites tapping an area of the screen or a button can invoke animations which we would like to benchmark.

BUG=321249

Review URL: https://codereview.chromium.org/105943004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240094 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240094