Implement the `require-sri-for` CSP directive

As defined in [1], this CSP directive allows developers to block resource
requests that do not contain integrity metadata. This includes contexts
like external scripts, workers, shared workers, service workers, external
stylesheets, preload requests, and requests originated by CSS @import.

[1]: https://w3c.github.io/webappsec-subresource-integrity/#opt-in-require-sri-for

Intent to implement: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/jyCdW1dHyYA/YefRSKs1AQAJ

BUG=618924
R=mkwst@chromium.org

Review-Url: https://codereview.chromium.org/2056183002
Cr-Commit-Position: refs/heads/master@{#405530}
42 files changed