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
Skip Interactive/spellcheck-paste-huge-text.html.

It has been timing out on the bots since it was added by r157747. Example:
http://build.chromium.org/p/chromium.perf/builders/Linux%20Perf%20%281%29/builds/26323/steps/blink_perf/logs/stdio

BUG=292481

Review URL: https://chromiumcodereview.appspot.com/23494052

git-svn-id: svn://svn.chromium.org/blink/trunk@157852 bbb929c8-8fbe-4397-9dbb-9b2b20218538