Bug 912666 - Introduce a 'scrollgrab' DOM property. r=roc,ehsan,Cwiiis
Bug 912666 - Propagate the 'scrollgrab' attribute to FrameMetrics. r=kats,Cwiiis
Bug 912666 - Force scroll frames for 'scrollgrab' elements to always create a real scrollable layer rather than a scroll info layer. r=roc,tn
Bug 912666 - Implement scroll grabbing in APZ. r=kats,Cwiiis