Bug 1949857 [wpt PR 50878] - Add CSS Color 4 gradient options to CanvasGradient, a=testonly

Automatic update from web-platform-tests
Add CSS Color 4 gradient options to CanvasGradient

Add ColorInterpolationMethod, HueInterpolationMethod and
premultipliedAlpha attributes to CanvasGradient as described here:

whatwg/html#7947

This is done behind the flag CanvasGradientCSSColor4, as spec is not yet
settled:

whatwg/html#10986

Chromestatus: https://chromestatus.com/feature/5168382242193408

I2P thread:
https://groups.google.com/a/chromium.org/g/blink-dev/c/6PhsIK2kxeM

Gradient::ColorInterpolation is also renamed to
Gradient::PremultipliedAlpha, because that's what it is.

Subpixel antialiasing for test labels is being inconsistently applied on
windows, so the tests are marked as failing with a bug.
crbug.com/396344375

Bug: 394052224
Change-Id: I6b1092376d08c051b161f83c82d585bd03cd4131
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6226021
Reviewed-by: Chris Harrelson <chrishtrchromium.org>
Commit-Queue: Aaron Krajeski <aaronhkchromium.org>
Cr-Commit-Position: refs/heads/main{#1423255}

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wpt-commits: fa9792e79e03bcdb6ce8a913c74981cbebe77756
wpt-pr: 50878

UltraBlame original commit: f95ce76846e9f8b4d9cce5a7926d6b2778608be2

Author: marco-c