Implement touch-action pan-x/pan-y handling on the main thread.

This extends the touch-action support on the main thread to handle and combine pan-x and pan-y values appropriately when computing the effective action to send to the embedder.

BUG=321643

Review URL: https://codereview.chromium.org/103823003

git-svn-id: svn://svn.chromium.org/blink/trunk@164108 bbb929c8-8fbe-4397-9dbb-9b2b20218538