The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8f672b21c1de610b8cc85790abbfcb592ab52ed7

commit 8f672b21c1de610b8cc85790abbfcb592ab52ed7
Author: Florin Malita <fmalita@chromium.org>
Date: Thu Jun 07 18:25:25 2018

Update gradient color stop serialization to match CSS Images4

https://github.com/w3c/csswg-drafts/issues/2714 clarified that
serialization should always use the expanded form.

Bug:  850537 
Change-Id: I77e60df149fb490b0c2c1db78f020a7246ea7b5c
Reviewed-on: https://chromium-review.googlesource.com/1090820
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565358}
[modify] https://crrev.com/8f672b21c1de610b8cc85790abbfcb592ab52ed7/third_party/WebKit/LayoutTests/fast/gradients/conic-gradient-parsing.html
[modify] https://crrev.com/8f672b21c1de610b8cc85790abbfcb592ab52ed7/third_party/WebKit/LayoutTests/fast/gradients/unprefixed-gradient-parsing-expected.txt
[modify] https://crrev.com/8f672b21c1de610b8cc85790abbfcb592ab52ed7/third_party/WebKit/LayoutTests/fast/gradients/unprefixed-gradient-parsing.html
[modify] https://crrev.com/8f672b21c1de610b8cc85790abbfcb592ab52ed7/third_party/blink/renderer/core/css/css_gradient_value.cc

Any chance to add WPTs for this instead of chromium-specific tests?

Yup, planning to add some serializing WPT tests before shipping.