Change WebView.zoomBy return type to void.

It turns out that the semantics of the return value are confusing
and so we decided to not return a result from this method at all.
The developer can call canZoomIn/canZoomOut to determine whether
the WebView is at the zoom limit, so there is no loss of
functionality.

BUG: 17374808
Change-Id: I4dfde71d5ac4a3f91c8755b257411aad06a1b1f6