Preparation of WebView for changes in the title bar.

Remove the zoom overview and 1x buttons from the ZoomButtonsController,
since the functionality will now be placed elsewhere.  Make zoomScrollOut
and canZoomScrollOut public (albeit hidden), so they can be called by the
Browser.  Also rename ExtendedZoomControls' mZoomControls to
mPlusMinusZoomControls so they are not confused with WebView's mZoomControls.