Remove webkit- prefix from classes used by View Source

Removing the webkit- prefix from viewsource as we do not want to keep it in blink.

BUG=378867

Review URL: https://codereview.chromium.org/306953006

git-svn-id: svn://svn.chromium.org/blink/trunk@178725 bbb929c8-8fbe-4397-9dbb-9b2b20218538