CSP: Source expressions can no longer lock sites into insecurity.

CSP's matching algorithm has been updated to make clever folks like Yan
slightly less able to gather data on user's behavior based on CSP
reports[1]. This matches Firefox's existing behavior (they apparently
changed this behavior a few months ago, via a happy accident[2]), and
mitigates the CSP-variant of Sniffly[3].

On the dashboard at https://www.chromestatus.com/feature/6653486812889088.

[1]: https://github.com/w3c/webappsec-csp/commit/0e81d81b64c42ca3c81c048161162b9697ff7b60
[2]: https://bugzilla.mozilla.org/show_bug.cgi?id=1218524#c2
[3]: https://bugzilla.mozilla.org/show_bug.cgi?id=1218778#c7

BUG=544765,558232

Review URL: https://codereview.chromium.org/1455973003

Cr-Commit-Position: refs/heads/master@{#360562}