Consolidate some text shadow code.

paintTextWithShadows had unused parameters, and identical logic for
constructing a text shadow draw looper existed in three places.

Review URL: https://codereview.chromium.org/356123002

git-svn-id: svn://svn.chromium.org/blink/trunk@177111 bbb929c8-8fbe-4397-9dbb-9b2b20218538