Update and fix sendBeacon() redirect behavior.

Refresh the implementation to follow the specification changes in

 https://github.com/w3c/beacon/pull/33
 https://github.com/w3c/beacon/pull/34

In particular, correctly flag a CORS-disallowed redirect as not to
be followed by WebURLLoader.

R=
BUG=628762

Review-Url: https://codereview.chromium.org/2177383006
Cr-Commit-Position: refs/heads/master@{#408380}
5 files changed