Remove duplicate RefPtr<> protect(this) in StyleSheetContents::checkLoaded()

This appears to have been introduced as a merge error in r128637.
The original fix that added first protector was in r127071.

Review URL: https://chromiumcodereview.appspot.com/23563003

git-svn-id: svn://svn.chromium.org/blink/trunk@156854 bbb929c8-8fbe-4397-9dbb-9b2b20218538