Support scrolling layers with the mousewheel
Bug:
6635437
Fix onGenericAction to support scrolling layers.
Calling startScrollingLayer detects if there is a layer under
the mouse pointer, and if so switches to DRAG_LAYER_MODE. Then, calling
doDrag will correctly handle scrolling the layer in DRAG_LAYER_MODE,
switching out of DRAG_LAYER_MODE when we hit the end of a layer, and
scrolling the page if there isn't a layer. It is the method used
by the touch drag path.
Change-Id: I00be09a63c40caa8fce94134c57db0894a62fef0