Bail out location.hash setter when set to the current hash

[domenic]

This adds a special case which is necessary for compatibility with deployed content, and implemented in 2/3 engines. Closes #7386.

• At least two implementers are interested (and none opposed):
  • Chromium and WebKit implement this already
  • Gecko (per What should happen when setting location.hash = location.hash? #7386)
• Tests are written and can be reviewed and commented upon at:
  • Test using location API to navigate to the current hash web-platform-tests/wpt#32291
• Implementation bugs are filed:
  • Chrome: N/A
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1733797
  • Safari: N/A

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

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