- Add TouchEvent and TouchEventPoint messages to the input event protocol.
- Extend the InputStub interface with an InjectTouchEvent() method.
- Update InputEventTracker to track touch points, and to cancel them
  when ReleaseAll() is called.
- Implement touch handling in the client plugin, but off by default.
  The web-app may choose to enable touch handling if desired.
- Add a TouchInputScaler that scales and clamps touch points, similarly to
  the MouseInputFilter, for use by both client and host.

BUG=314515

Review URL: https://codereview.chromium.org/799233004

Cr-Commit-Position: refs/heads/master@{#318284}