Refactoring more webview zoom code into ZoomManager.

This CL is #2 of multiple CL's to remove the zoom code
from webview and place it into webkit.ZoomManager. Each
commit is only part of the entire refactoring so there
are many ZoomManager.* variables still referenced from
WebView.

Change-Id: I21f6517dff46b65a277bc67120ffabe043098e5e
http://b/2671604
4 files changed