Remove new browsing context/Document referrer assignment

[domfarolino]

This implements part of the proposed plan in #10099, which notes that only Chrome implements step 16.1 of "create a new browsing context and document", which assigns the brand new about:blank Document's referrer to its creator's origin, if it exists. This PR updates the spec to match Safari and Firefox, and Chrome's behavior will change accordingly.

• At least two implementers are interested (and none opposed):
  • Chrome
  • Safari (already implements)
  • Gecko (already implements)
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium:
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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/document-sequences.html ( diff )

[domfarolino]

PTAL, but I've added the DNM label on this until:

• I figure out which tests to already exists to link here, or which ones I need to implement
• File a Chromium bug