AMP pages are loaded in an iframe, and scroll events that occur there are not forwarded to the UIScrollView delegate.  As a result, toolbars cannot be scrolled into our out of view when an AMP page is loaded.  We should update FullscreenModel to accept alternative input events to enable Fullscreen for these pages.

Marking this as M70, as we won't have a chance to experiment with this in the M69 timespan.