Progress on HDR canvas prototype

I've sent out the "intent to prototype" for HDR canvas to blink-dev (this
was a while ago at this point).

It is now possible to create a canvas that has color space "srgb-linear",
"rec2100-hlg" and "rec2100-pq". These color spaces also work for WebGL. The
latter two spaces should have appropriate tone mapping applied (though not
always, the hiccups in the path are being worked through). There is also
ongoing work to ensure that importing HDR images and videos takes the
appropriate color conversion path. There is a test of these paths
available at:
https://ccameron-chromium.github.io/webgl-examples/canvas-hdr.html