Bug 921942 - Broadcast scroll positions r=yoric

From 5f535195e10d6cccbedbdf607ff194450a40c4ed Mon Sep 17 00:00:00 2001
Bug 921942 - Add FrameTree and scroll position broadcast tests r=yoric

From 5645706beeae13673e87ba665f225e01a32949e2 Mon Sep 17 00:00:00 2001