New WebView input dispatcher.

Bug: 6317798

Stuff that's better:

1. We maintain two queues in a way that ensures that WebView and
WebKit both see consistent streams of events, even in cases
where WebKit times out.  We send ACTION_CANCEL if necessary, etc.

2. All pointer events go through the same channel, including
hover and click ("touch up") events, to ensure correct ordering.

3. Given that the input events are in a separate queue, we can
force execution of all of these events whenever we like, making new
latency optimizations possible.

4. The entire history of each touch event is sent to the web
application to enable smoother interaction.

5. The web application may choose to intercept a touch event stream
at any time by issuing "prevent default".  Previously, it could only
prevent default on the initial down event.  The new behavior is more
standards compliant.

Change-Id: I42d2d045e7d44af7c54b29570f188b7400d91d4e