Webkit: Change thread priority to improve response

Change-Id: I0512ad0d4b6e3beb86417b0d6af24c47b86a84f8
[webkit] Postpone sending scroll message to webcore thread

Post a message to the UI thread instead, to send the
scroll message later. Thus reduce the computation until
currnet scrolling finishes

Change-Id: Iff94af37e9070e63e0797195c7588343ab077bb3
Webkit: Specific action if DoubleTap on a known element

DoubleTap in empty space will zoom like before
DoubleTap on a known element will notify the app with the specific action message.

Change-Id: Ib6574dbe9254d0fc7278b947e2ff3b845d26b899

Change-Id: Ibf405d6a0d9b495778c4a44956816b06c6321a3f