Fix wasteful ScrollAnimator creation in EventHandler::handleGestureShowPress

This replaces a call to ScrollableArea::scrollAnimator (which creates a
ScrollAnimator if one doesn't already exist) with a call to
ScrollableArea::existingScrollAnimator (which returns null if one doesn't
already exist). Since EventHandler::handleGestureShowPress is just trying
to cancel any in-progress animations, there's no need to create a
ScrollAnimator if one doesn't already exist.

Review URL: https://codereview.chromium.org/386703003

git-svn-id: svn://svn.chromium.org/blink/trunk@177893 bbb929c8-8fbe-4397-9dbb-9b2b20218538