Set default synthetic gesture source type on aura to be touch event

Since we care touch scroll latency more than mouse wheel scroll latency,
lets change the default gesture source type to be touch.
If we need to monitor mouse wheel scroll latency in the future, we can
then setup a new perf bot running telemetry smoothness test with mouse
event selected.

BUG=246034
TEST=telemetry smoothness test on CrOS now shows mean_touch_scroll_latency
     instead of mean_mouse_wheel_scroll_latency.

Review URL: https://codereview.chromium.org/164023004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251962 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251962