Fix touch hit rect calculation for elements inside scrolled layers.

When starting touch hit rect calculation we compute the initial offset of a touch-handler element from it's enclosing layer.  We expect to have the offset in the layer's coordinate space, but instead we were using the offset from the element itself, and so incorrectly subtracting any scroll offset it might have.  RenderObject::offsetFromAncestorContainer doesn't give us any mechanism to specify which of these two behaviors we want, so fix this by just adding any scrolledContentOffset back in the intial offset calculation.

BUG=287885

Review URL: https://chromiumcodereview.appspot.com/23707027

git-svn-id: svn://svn.chromium.org/blink/trunk@157479 bbb929c8-8fbe-4397-9dbb-9b2b20218538