Refactor zoom logic from WebView to ZoomManager.

This CL is one in a series of CL's intended to remove the zoom
logic from WebView. This CL focuses on refactoring the initialScale
variable as well as the onSizeChanged() and NEW_PICTURE_MSG portions
of the code.

Change-Id: Id44bce50378aa7cdfc1c8110818e18500679c517
http://b/2671604