Prevent intermediate scale updates from being sent to WebKit while zooming.

The addition of the HW accelerated logic causes us to manipulate the
zoom scale factor in the zoom manager two additional times. These
manipulations occur after the mZoomScale has been set to zero is how we
previously tested to see if a fixed length animation was occuring.

bug: 3451126
Change-Id: If2992adbe36fa471bb1bb5013495e1adc74b5fab