Verify that @font-face fallback fonts can't be probed

In #8455 (closed), I disabled the local fallback rendering for @font-face. I did not attempt to disable all of the code that handles cases where WebFonts weren't downloaded yet, because I was hesitant to introduce bugs, especially without a test case to verify that what I disabled was actually relevant to font probing.

However, there did appear to be sections of the code that at least internally allowed Firefox to estimate font sizes before the webfont completed loading. It was not immediately clear to me if these estimations were available to content or not, and which fonts they were based on.

If they are available, it would be very useful to have a test case to verify against as I try removing/altering some of those codepaths.