Update: Trying to add color management for to toBlob/toDataURL, I encountered some difficulties with how Skia encoders encode the wide color gamut images. Apparently, this is what happening (@brianosman, please correct me if I'm wrong):

The image is encoded in wide color gamut, 8-8-8-8 pixel format, sRGB transfer curve.

This behavior makes the color conversion go in flux somewhere in the code path, causing double premul when the image is drawn onto the canvas in RecordPaintCanvas (which is part of the test scenario designed to check the code correctness).

@brianosman believes that we still can get the encoders to do the right thing, if we do it carefully. I tend to agree with him, even though I still don't know what I'm doing wrong.

https://chromium-review.googlesource.com/c/chromium/src/+/824462 fixes this issue.