Ensure WebView passes a complete/in-order event stream to

ScaleGestureDetector. Prerequisite for several ScaleGestureDetector
bugfixes.

Bugfixes for ScaleGestureDetector.

Make ScaleGestureDetector properly support >2 pointers. Track the most
recent two pointers for scale gestures.

Change-Id: I966319ad438c1c8e1103f245b53b3862c2d4e3c0