The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2b63d92e98cfdb984a8aab368c4586e2ac122aa9

commit 2b63d92e98cfdb984a8aab368c4586e2ac122aa9
Author: Darren Shen <shend@chromium.org>
Date: Sat Jan 20 06:42:15 2018

[css-typed-om] Update paint style map behaviour for invalid properties.

Custom paint callbacks receive a computed style map (implemented as
FilteredComputedStylePropertyMap) that contain only properties specified
in inputProperties(). For forwards-compatiblity, inputProperties() is
allowed to have "invalid" or shorthand properties, which are not allowed
to be in the styleMap.

As resolved in [1], the style map should ignore any invalid/shorthand
properties. Furthermore, if a custom property is specified in
inputProperties(), it should have a default value of CSSUnparsedValue()
(not "null").

These changes also require changes in custom paint tests. Namely:

- Can no longer get "border-radius" from the style map. Have to use
  a longhand property like "border-left-radius".

- Getting custom properties like "--bar" will have a default value of
  CSSUnparsedValue.

[1] https://github.com/w3c/css-houdini-drafts/issues/523

Bug:  803690 
Change-Id: I148b52568de698f69ee0bf594a2b92a9751ea484
Reviewed-on: https://chromium-review.googlesource.com/874984
Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530763}
[modify] https://crrev.com/2b63d92e98cfdb984a8aab368c4586e2ac122aa9/third_party/WebKit/LayoutTests/external/wpt/css/css-paint-api/style-background-image.https.html
[modify] https://crrev.com/2b63d92e98cfdb984a8aab368c4586e2ac122aa9/third_party/WebKit/LayoutTests/external/wpt/css/css-paint-api/style-before-pseudo.https.html
[modify] https://crrev.com/2b63d92e98cfdb984a8aab368c4586e2ac122aa9/third_party/WebKit/Source/core/css/CSSVariableData.h
[modify] https://crrev.com/2b63d92e98cfdb984a8aab368c4586e2ac122aa9/third_party/WebKit/Source/core/css/cssom/CSSUnparsedValue.cpp
[modify] https://crrev.com/2b63d92e98cfdb984a8aab368c4586e2ac122aa9/third_party/WebKit/Source/core/css/cssom/CSSUnparsedValue.h
[modify] https://crrev.com/2b63d92e98cfdb984a8aab368c4586e2ac122aa9/third_party/WebKit/Source/core/css/cssom/FilteredComputedStylePropertyMap.cpp
[modify] https://crrev.com/2b63d92e98cfdb984a8aab368c4586e2ac122aa9/third_party/WebKit/Source/core/css/cssom/FilteredComputedStylePropertyMapTest.cpp