WebView.mHeldMotionless should only be set to MOTIONLESS_FALSE
if some scrolling occurs, even in the case where motion is discarded
by snap-scroll mode.
Refactoring the code allows the code to be smaller, and provides
a single point where scrollBy() is called while dragging.
This doesn't change the snap-scroll logic in any way.
related to http://b/issue?id=2092446