Skia is deprecation the use of Unpremultiplied alpha with SkImage.  Soon, the Unpremultiplied alpha type will only be supported by readPixels/writePixels.  The conversion performed by createImageBitmap therefore need to be re-written.

The plan is to use a new API SkImage::makeScaled that will take care of resizin, format conversion and colorspace conversion.
https://skia-review.googlesource.com/c/7222/

Cropping can be performed using SkImage::makeSubset

The notion of whether pixel values are unpremultipled will have to be stored outside of SkImage (a member of ImageBitmap).  SkImage will still be able to store Unpremultiplied pixel values, but it will not do the right thing for drawing to an SkCanvas, so it will be necessary to bootstrap a premul conversion to support that use case.