All layout tests currently composite to sRGB.

There exist two mechanisms to specify a color profile, and neither of them affect compositing. They are:
- The Javascript setColorProfile method
- The command line flag --force-color-profile=

End-to-end testing of canvases that are wider gamut than sRGB using layout tests will require that compositing not crush all colors to sRGB.

Using the JS method for this would probably be the easiest (for one or two tests), but we'll need to make sure it's not racey.