Ensure only one scroll child per RLB-maintained GraphicsLayer subtree

Previously, we'd simply set the scroll parent association for the
childForSuperlayers. Problem is, the childForSuperlayers could change
(we could add an ancestor clipping layer, for example), and if it did,
the old scroll child would still think that it had a scroll parent. This
CL ensures that scroll child/parent associations are cleared as
necessary.

R=hartmanng,shawnsingh
BUG=None

Review URL: https://chromiumcodereview.appspot.com/24247007

git-svn-id: svn://svn.chromium.org/blink/trunk@158292 bbb929c8-8fbe-4397-9dbb-9b2b20218538