MotionMark webgl benchmark doesn't do anything (offsetWidth/offsetHeight of body is 0)

[jdm]

https://browserbench.org/MotionMark/developer.html?test-interval=30&display=minimal&tiles=big&controller=ramp&frame-rate=50&kalman-process-error=1&kalman-measurement-error=4&time-measurement=performance&suite-name=3DGraphics&test-name=WebGL

[jdm]

I've verified that it's running the animate function from https://searchfox.org/mozilla-central/source/third_party/webkit/PerformanceTests/MotionMark/tests/3d/resources/webgl.js#135, so I'm not really sure what's going wrong.

[jdm]

This is caused by this snippet during the test initialization:

        this._element.setAttribute("width", document.body.offsetWidth);
        this._element.setAttribute("height", document.body.offsetHeight);

In Servo document.body.offsetWidth and document.body.offsetHeight return 0, which sets the canvas element's logical width and height to 0, therefore displaying nothing. Sigh.