Make |node| a RefPtr in EventHandler::handleGestureScrollEnd()

It is possible for |m_scrollGestureHandlingNode| to be destroyed before
we attempt to use |node|, making |node| a dangling pointer and causing
a use-after-free error. Make |node| a RefPtr to avoid this.

BUG=275970

Review URL: https://codereview.chromium.org/26277002

git-svn-id: svn://svn.chromium.org/blink/trunk@159919 bbb929c8-8fbe-4397-9dbb-9b2b20218538