Fix value of pageScaleLayer registered with CC.

The current value of the page scale layer being registered with CC is incorrect; instead of passing the root layer, we should be passing the parent of the root scroll layer.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23530061

git-svn-id: svn://svn.chromium.org/blink/trunk@157850 bbb929c8-8fbe-4397-9dbb-9b2b20218538