Clean up remaining uses of WebCore namespace

This CL removes uses of the WebCore namespace that have crept in during the
transition.

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/426863003

git-svn-id: svn://svn.chromium.org/blink/trunk@179104 bbb929c8-8fbe-4397-9dbb-9b2b20218538