These fields were accidentally exposed to the web in crrev.com/562990 (via crrev.com/427153). They are not part of any specification. These fields can only degrade the quality of the output image (because all OffscreenCanvas elements are 8-bit sRGB or Display P3).
The behavior caused by these fields at odds with the HTML specification, section "Serializing bitmaps to a file", which says that For image types that support color profiles, the serialized image must include a color profile indicating the color space of the underlying bitmap. For example, for a Display P3 canvas, calling convertToBlob with no arguments should result in a blob that specifies a Display P3 color profile. When this feature is removed, the images resulting from convertToBlob are guaranteed to be an equal or higher fidelity representation as compared with behavior before this feature was removed, because all OffscreenCanvas elements are 8-bit sRGB or Display P3.