We need layout tests with premultiply="none", color components > 0 and alpha = 0 for color-managed and non-color-managed code path.

Fixing scaling requires Skia to support unpremul mode in SkImage::scalePixels().

Unpremul ImageBitmap resize code path in Blink should respect non-zero color components with zero alpha value when the option premultiplyAlpha is set to "none". This requires skia:5733 to get fixed. Meanwhile, I can think of two workarounds:

1. Keep RGB values intact, map alpha from 0-255 to, like, 32-255:
Map(A) = (A+32)*255/(255+32)
Code path: Map(A), premul, scale, unpremul, Unmap(A)

2. Do the scaling twice. Break RGBA to RGB/255 and A/255,255,255. This workaround does not need additional premul/unpremul step, but requires additional memory.
Code path: Break(RGB,A), scale(RGB), scale(A), Merge(RGB,A).

Thoughts?

Regarding comment 5: The first solution also requires a copy of pixels as Map/Unmap cannot happen on the original data (as we might end up modifying alpha in the round-trip).

Option 1) is unacceptably lossy IMHO, unless you use an FP16 intermediate. In that case, might as well do 2).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ec67cae3b77ca0f9d641a2722d8d1f6090b346fd

commit ec67cae3b77ca0f9d641a2722d8d1f6090b346fd
Author: Reza.Zakerinasab <zakerinasab@chromium.org>
Date: Mon Nov 20 17:54:39 2017

With {premultiplyAlpha: "none"} ImageBitmap should avoid premul code path

When premultiplyAlpha is set to "none" in ImageBitmapOptions, we should never pass through premul
format in color managed ImageBitmap code.

Bug:  781908 ,  785313 
Change-Id: I3a2fc1664dd57530bdc31694c5f9d61e95a9501b
Reviewed-on: https://chromium-review.googlesource.com/759116
Commit-Queue: Mohammad Reza Zakerinasab <zakerinasab@chromium.org>
Reviewed-by: Justin Novosad <junov@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517858}
[modify] https://crrev.com/ec67cae3b77ca0f9d641a2722d8d1f6090b346fd/third_party/WebKit/Source/core/html/ImageData.cpp
[modify] https://crrev.com/ec67cae3b77ca0f9d641a2722d8d1f6090b346fd/third_party/WebKit/Source/core/html/ImageData.h
[modify] https://crrev.com/ec67cae3b77ca0f9d641a2722d8d1f6090b346fd/third_party/WebKit/Source/core/html/ImageDataTest.cpp
[modify] https://crrev.com/ec67cae3b77ca0f9d641a2722d8d1f6090b346fd/third_party/WebKit/Source/core/imagebitmap/ImageBitmap.cpp
[modify] https://crrev.com/ec67cae3b77ca0f9d641a2722d8d1f6090b346fd/third_party/WebKit/Source/core/imagebitmap/ImageBitmap.h
[modify] https://crrev.com/ec67cae3b77ca0f9d641a2722d8d1f6090b346fd/third_party/WebKit/Source/modules/canvas/canvas2d/BaseRenderingContext2D.cpp