CSP: Fix protocolless 'frame-ancestor' source expressions.

When checking 'frame-ancestor', we end up asking for the SecurityOrigin
of the protected resource before we actually have a protected resource
whose origin we could ask for. This, unsurprisingly, crashes.

This patch adds a protocol property to the ContentSecurityPolicy object
so that we can perform all the checks for source expressions that don't
specify protocols without crashing the renderer.

BUG=424074
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/658073002

git-svn-id: svn://svn.chromium.org/blink/trunk@183801 bbb929c8-8fbe-4397-9dbb-9b2b20218538