Make reftests wait for fonts to load over WebDriver

[gsnedders]

Fixes #6328. Fixes #5412. Fixes, partially, #6327.

This doesn't change the Martionette script as per w3c/csswg-drafts#1088 (comment) Firefox blocks the load event on fonts loading.

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Firefox (nightly)

Testing web-platform-tests at revision 9d65700
Using browser at version BuildID 20170714100217; SourceStamp 67cd1ee26f2661fa5efe3d952485ab3c89af4271
Starting 10 test iterations
No tests run.

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Sauce (safari)

Testing web-platform-tests at revision 9d65700
Using browser at version 10.0
Starting 10 test iterations
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Chrome (unstable)

Testing web-platform-tests at revision 65e7d89
Using browser at version 61.0.3153.4 dev
Starting 10 test iterations
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Sauce (MicrosoftEdge)

Testing web-platform-tests at revision 65e7d89
Using browser at version 14.14393
Starting 10 test iterations
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[bobholt]

This looks good to me, but I'd like to wait for @jgraham to weigh in on how we make sure it doesn't break Servo.

[jdm]

These changes will not break Servo.

[sideshowbarker]

w3c-test:mirror

[jgraham]

Reviewed 6 of 6 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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