CSP: Use a specified frame for reporting 'frame-ancestors' violations.

We don't have an ExecutionContext when dealing with 'frame-ancestors'
violations. This patch uses the LocalFrame which we pass into
allowAncestors in order to provide hooks for both PingLoader and
ConsoleMessages.

It also fiddles a bit with the reporting data, as we'd otherwise suggest
that 'about:blank' failed to load (as that's the initial contents of a
frame before we navigate to a document).

BUG=412725

Review URL: https://codereview.chromium.org/553423002

git-svn-id: svn://svn.chromium.org/blink/trunk@181910 bbb929c8-8fbe-4397-9dbb-9b2b20218538