Fix overlay scrollbars not disappearing on Mac.

This was a regression introduced by:
https://chromiumcodereview.appspot.com/16336016

That CL made ScrollElasticityController::handleWheelEvent() sometimes
return false, which resulted in didEndScrollGesture() not being called
in all cases, which is needed to fade out overlay scroll bars.

Unfortunately, not enough of the Mac overlay scroll bar code runs in
layout tests for this to be testable. 

BUG=251910
TEST=See bug.

Review URL: https://chromiumcodereview.appspot.com/19697019

git-svn-id: svn://svn.chromium.org/blink/trunk@154807 bbb929c8-8fbe-4397-9dbb-9b2b20218538