Go through root to get loading clients in StyleSheetContents::checkLoaded.

Added an assertion to the patch in https://codereview.chromium.org/444223004/

In StyleSheetContents::checkLoaded after parentStyleSheet check, "this pointer" is guaranteed to be equal to rootStyleSheet.
So we can only use this pointer instead of rootStyleSheet.

Review URL: https://codereview.chromium.org/456673002

git-svn-id: svn://svn.chromium.org/blink/trunk@179904 bbb929c8-8fbe-4397-9dbb-9b2b20218538