Fix rubber-band overhang getting stuck after a fling.

This got broken by: https://chromiumcodereview.appspot.com/24284003

That CL made the code not start the timer under certain cases
when PlatformWheelEventPhaseEnded was received.

This caused a problem with momentum scroll. How that works is
after the regular scroll events are received, including the
PlatformWheelEventPhaseEnded event, then momentum scroll
events start appearing.

The momentum events can then cause overscroll when a fling
happens. However, the cleanup after that was expected to
happen in the snapRubberBand timer launched from the phase
end event (which now wouldn't be getting launched).

This CL fixes the issue by starting the snap rubber band
timer when a momentum scroll starts (if it hasn't been
started during the phase end event).

BUG=290590

Review URL: https://codereview.chromium.org/24486006

git-svn-id: svn://svn.chromium.org/blink/trunk@158576 bbb929c8-8fbe-4397-9dbb-9b2b20218538