CSP: Update violation report 'Content-Type' header.

The spec recently changed the type of violation reports from
'application/json' to 'application/csp-report'[1]. This patch implements
that change by adding a type flag to the relevant PingLoader method.

[1]: https://dvcs.w3.org/hg/content-security-policy/rev/9c1ebbb88a76

Review URL: https://chromiumcodereview.appspot.com/19091002

git-svn-id: svn://svn.chromium.org/blink/trunk@154215 bbb929c8-8fbe-4397-9dbb-9b2b20218538