GitLab

Align implementation with spec change https://github.com/whatwg/fetch/pull/437

PSA at https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/BN6tqGLBmuI

BUG=698521

Review-Url: https://codereview.chromium.org/2732853003
Cr-Commit-Position: refs/heads/master@{#455723}