First draft of multitouch in the WebView.

Currently we just handle a simple pinch action. We
will wait for framework support for more complicated
gesture.

When pinch in the webview, zoom level will be changed
on the fly. But we won't re-wrap the text until user
action like double tap, rotate screen.

Double tap will re-layout the page and wrap the text
to the screen width. We try to keep the spot you
tapped at the same place on the screen after relayout.
If the block after relayout fully fit on the current
screen, we will center it for easy reading.

Fix http://b/issue?id=2360032