Gecko work for bug 823619: Dispatch spec-compliant mouse events when touch events are available. r=cjones,jlebar,schien,vingtetun a=blocking-basecamp

This is a rollowup of the following patches
Bug 823619, part 1: Make TabChild dispatch spec-compliant compat mouse events. r=mwu
Bug 823619, part 2: Use touch event for scrolling if available. r=cjones,schien,vingtetun a=blocking-basecamp