Further tabIndex getter tweaks

[domenic]

Further code inspection and test-writing revealed that SVG <a> elements
get a 0 default tabIndex in all engines, and <embed> and <object>
elements get a 0 default tabIndex in Blink and WebKit. These were
omitted in 15cf23a.

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• At least two implementers are interested (and none opposed):
  • Blink and WebKit already implement everything here; Gecko implements the SVG a portion
• Tests are written and can be reviewed and commented upon at: HTML: WPT for tabIndex web-platform-tests/wpt#17657
• Implementation bugs are filed:
  • Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=982195, https://bugs.chromium.org/p/chromium/issues/detail?id=982350
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1564541
  • Safari: https://bugs.webkit.org/show_bug.cgi?id=199606

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

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Honestly I merged #4754 prematurely. Given the rapidly-evolving state of my knowledge, and the refactoring I had to do for details which didn't get editorial review, I should not have merged that. My apologies. I can fix anything else wrong with #4754 here, as necessary.

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/infrastructure.html ( diff )
/interaction.html ( diff )

[rniwa]

I don't think was right change. Neither Firefox 67 or 68, and Safari 12 & 13 both uses -1 for the default tab index of object. Only Chrome uses the default tab index of 0.