Bug 886576 - Fix screen-sized pages not revealing the toolbar. r=kats

If a page changed size to be the exact size of the screen, the toolbar would
not be revealed.
Bug 892246 - Allow subdocument scrolling to reveal margins. r=kats

This adds a notification callback to PanZoomTarget that the PanZoomController
can call to notify GeckoLayerClient that a subdocument is being scrolled. This
allows GeckoLayerClient to call LayerMarginsAnimator and alter the margins
accordingly, stopping a page from trapping the toolbar on/off the screen with
a screen-covering subframe.