Add touch scrolling modes experimental flags

In particular, adds --touch-scrolling-mode=absorb-touchmove,
and --touch-scrolling-mode=sync-touchmove.

Split out of https://codereview.chromium.org/156783006, which was migrated from https://codereview.chromium.org/140253005/ so some review history is there.

BUG=328503
TEST=TouchEventQueueTest.SyncTouchMoveDoesntCancelTouchOnScroll,
  TouchEventQueueTest.TouchAbsorption

Review URL: https://codereview.chromium.org/166923002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251596 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251596