Slow performance in webrender-demos/spheres.html

[glennw]

Test case: https://github.com/pcwalton/webrender-demos/blob/master/spheres.html

The frame rate in the spheres demo is much lower than it used to be.

After some profiling, it seems that the rendering performance is not the bottleneck:

Resizing the window to be very small has no effect on the framerate, ruling out fill rate as a bottleneck.

I recorded a flame graph profile of the CPU time:

The majority of the CPU time appears to be spent in layout in this test case - the biggest factor is in style recalc. After layout, the next biggest consumer of CPU time is the calls in the script thread (set_width, set_height, set_top, set_left).

[glennw]

cc @metajack