Bug 944047 - Change the initialisation order in BrowserElementPanning.js r=cwiiis

Change the order of initialisation in BrowserElementPanning.js so that
TabChild::IsAsyncPanZoomEnabled isn't called before
TabChild::InitRenderingState.
Bug 944047 - Capture scroll events in TabChild. r=kats

Set useCapture to true in the event listener on scroll events. It appears that
scroll events for sub-frames are often(/always?) swallowed before they reach
this point.
Bug 944047 - Store the last FrameMetrics for sub-frames in TabChild. r=botond

As well as the root frame, make sure to store the last received FrameMetrics
for sub-frames in TabChild. This prevents us sending scroll events triggered
by the APZC back to the APZC.