The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c80ecf5d3c1a8bc59a3f0c6bd9bfe226c316cc02

commit c80ecf5d3c1a8bc59a3f0c6bd9bfe226c316cc02
Author: Anders Hartvoll Ruud <andruud@chromium.org>
Date: Sat Jul 14 23:02:53 2018

[css-properties-values-api] Support repeated values in Typed OM.

This patch fixes two similar problems:

 1. Computed list-valued properties show up as a CSSStyleValue when calling
    computedStyleMap().get(name).
 2. Computed list-valued properties show up as a CSSStyleValue when calling
    computedStyleMap().getAll(name).

The fixes are also similar:

 1. StylePropertyMapReadOnly::get only checked property.IsRepeated() when
    determining whether it should look inside a possible list. However,
    CSSPropertyVariable is never repeated. This is because
    CSSPropertyVariable does not represent a single, specific custom
    property, but rather _all_ custom properties.

    For CSSPropertyVariable, if we have a CSSValueList, it means that
    it is registered with either '<foo>+', or '<foo>#', and that makes it
    a "list valued property" [1].

2. Similar to (1), the value would be treated as unsupported if property.
   IsRepeated() was false. Again, a situation of CSSPropertyVariable+
   CSSValueList means that the property is registered as a list.

Note1: The value for a custom property can only be a CSSValueList if the
       runtime enabled feature CSSVariables2 is enabled. In all other
       cases, it's a CSSCustomPropertyDeclaration. See
       ComputedStyleCSSValueMapping::Get ('registry' is nullptr without
       the feature).

Note2: It might make more sense to ignore the repeatability of the
       property, and instead just look at whether we actually have a
       CSSValueList, but such a change would require a change of the
       css-typed-om spec.

[1] https://drafts.css-houdini.org/css-typed-om-1/#list-valued-properties
    https://drafts.css-houdini.org/css-typed-om-1/#reify-stylevalue

R=chrishtr@chromium.org

Bug:  862152 
Change-Id: Ic854f54f14313b6b683601d0677dcc1dd4205ccd
Reviewed-on: https://chromium-review.googlesource.com/1136295
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575166}
[modify] https://crrev.com/c80ecf5d3c1a8bc59a3f0c6bd9bfe226c316cc02/third_party/WebKit/LayoutTests/external/wpt/css/css-properties-values-api/typedom.tentative.html
[modify] https://crrev.com/c80ecf5d3c1a8bc59a3f0c6bd9bfe226c316cc02/third_party/blink/renderer/core/css/cssom/style_property_map_read_only.cc
[modify] https://crrev.com/c80ecf5d3c1a8bc59a3f0c6bd9bfe226c316cc02/third_party/blink/renderer/core/css/cssom/style_value_factory.cc