Remove support for prefixed Content Security Policy headers.

This patch retains deprecation warnings for 'X-WebKit-CSP-Report-Only'
and 'X-WebKit-CSP' headers, but turns off their functionality. We'll
now inform developers that the headers are unsupported, and direct them
to the canonical headers going forward.

The warnings will be removed in a future patch, hopefully in the next
release cycle.

blink-dev discussion: https://groups.google.com/a/chromium.org/d/msg/blink-dev/3AaCbkdGSPs/pnQO579qrNkJ

BUG=307404

Review URL: https://codereview.chromium.org/27314002

git-svn-id: svn://svn.chromium.org/blink/trunk@159714 bbb929c8-8fbe-4397-9dbb-9b2b20218538