Timeline: add layer root node reference to ScrollLayer event

ScrollLayer is emitted when we do a (slow) blink-side scroll, so help users trace it down to the layer that caused this.

Review URL: https://chromiumcodereview.appspot.com/18079002

git-svn-id: svn://svn.chromium.org/blink/trunk@154572 bbb929c8-8fbe-4397-9dbb-9b2b20218538