Update XMLHttpRequest forbidden header names.

See http://fetch.spec.whatwg.org/#forbidden-header-name .

BUG=373120

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=177762

Review URL: https://codereview.chromium.org/378183003

git-svn-id: svn://svn.chromium.org/blink/trunk@178023 bbb929c8-8fbe-4397-9dbb-9b2b20218538