Refactor appendAuthorStyleSheets and appendPendingAuthorStyleSheets.

appendAuthorStyleSheets and appendPendingAuthorStyleSheets have the same code in thier for loops.
We should create a helper method and use it.

BUG=320929

Review URL: https://codereview.chromium.org/76143002

git-svn-id: svn://svn.chromium.org/blink/trunk@162461 bbb929c8-8fbe-4397-9dbb-9b2b20218538