Drop [TreatNullAs=NullString] for Document.cookie

https://html.spec.whatwg.org/#document

After this change, setting document.cookie to null will stringify to
"null", so it's simply like setting a cookie by that name. This matches
Firefox Nightly and Edge.

BUG=497307

Review URL: https://codereview.chromium.org/1458753003

Cr-Commit-Position: refs/heads/master@{#360567}
3 files changed