This is because this returns null even though we're in a <browser>.

And remove realFrameElement while at it, since it now should work for
those cases too. I audited callers in devtools/, toolkit/ and browser/
and they all should be ok with this change.

There are Thunderbird tests testing this, see the comments at the end of
the regressing bug.

I think this is better than doing a whack-a-mole changing frameElement
to window.browsingContext?.embedderElement, but lmk if you just want me
to fix the LayoutUtils.jsm caller instead.

Backed out for causing mochitest chrome failures on test_focusrings.xhtml.

Push with failures

Failure log

Backout link

[task 2022-01-19T01:40:40.557Z] 01:40:40     INFO - TEST-PASS | dom/tests/mochitest/general/test_focusrings.xhtml | appearance on list after tab focus 
[task 2022-01-19T01:40:40.558Z] 01:40:40     INFO - Buffered messages finished
[task 2022-01-19T01:40:40.559Z] 01:40:40     INFO - TEST-UNEXPECTED-FAIL | dom/tests/mochitest/general/test_focusrings.xhtml | mouse click on <button id='elem' class='canfocus'>Button</button> - got [object HTMLBodyElement], expected [object HTMLButtonElement]
[task 2022-01-19T01:40:40.559Z] 01:40:40     INFO - SimpleTest.is@chrome://mochikit/content/tests/SimpleTest/SimpleTest.js:500:14
[task 2022-01-19T01:40:40.559Z] 01:40:40     INFO - testHTMLElements@chrome://mochitests/content/chrome/dom/tests/mochitest/general/test_focusrings.xhtml:171:7
[task 2022-01-19T01:40:40.559Z] 01:40:40     INFO - testMacFocusesFormControl@chrome://mochitests/content/chrome/dom/tests/mochitest/general/test_focusrings.xhtml:109:19
[task 2022-01-19T01:40:40.559Z] 01:40:40     INFO - pushPrefEnv@resource://specialpowers/SpecialPowersChild.jsm:899:13
[task 2022-01-19T01:40:40.559Z] 01:40:40     INFO - Not taking screenshot here: see the one that was previously logged
[task 2022-01-19T01:40:40.560Z] 01:40:40     INFO - TEST-UNEXPECTED-FAIL | dom/tests/mochitest/general/test_focusrings.xhtml | mouse click on <button id='elem' class='canfocus'>Button</button> ring - got "0px", expected "2px"
[task 2022-01-19T01:40:40.561Z] 01:40:40     INFO - SimpleTest.is@chrome://mochikit/content/tests/SimpleTest/SimpleTest.js:500:14
[task 2022-01-19T01:40:40.561Z] 01:40:40     INFO - testHTMLElements@chrome://mochitests/content/chrome/dom/tests/mochitest/general/test_focusrings.xhtml:172:7
[task 2022-01-19T01:40:40.561Z] 01:40:40     INFO - testMacFocusesFormControl@chrome://mochitests/content/chrome/dom/tests/mochitest/general/test_focusrings.xhtml:109:19
[task 2022-01-19T01:40:40.561Z] 01:40:40     INFO - pushPrefEnv@resource://specialpowers/SpecialPowersChild.jsm:899:13
[task 2022-01-19T01:40:40.561Z] 01:40:40     INFO - TEST-PASS | dom/tests/mochitest/general/test_focusrings.xhtml | mouse click on <button id='elem' class='canfocus'>Button</button> selector 

https://hg.mozilla.org/mozilla-central/rev/f16516a913a0