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