Any update here?

Change up for review at https://codereview.chromium.org/2089593002/ - just had to dedicate some time to resolving merge conflicts and adding 'const' in all the bits it's now needed!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6b045cf7e0fe8f7eb94dc94508d75dbde7e63365

commit 6b045cf7e0fe8f7eb94dc94508d75dbde7e63365
Author: andymutton <andymutton@chromium.org>
Date: Wed Jun 29 00:42:45 2016

Expand custom property shorthands to longhand with pending value.

Introduces CSSPendingSubstitutionValue for each longhand, containing a
reference back to the shorthand property.

Changes CSSVariableResolver to just resolve variables, rather than
resolving and applying them (a task that StyleBuilderCustom can pretty
easily take care of).

Introduces a property cache on StyleResolverState to limit the amount
of parsing required during resolution. This is somewhat unattractive to
me, but there was a lack of better alternatives.

BUG= 612634 

Review-Url: https://codereview.chromium.org/2089593002
Cr-Commit-Position: refs/heads/master@{#402620}

[add] https://crrev.com/6b045cf7e0fe8f7eb94dc94508d75dbde7e63365/third_party/WebKit/LayoutTests/fast/css/variables/longhand-pending-shorthand-substitution.html
[modify] https://crrev.com/6b045cf7e0fe8f7eb94dc94508d75dbde7e63365/third_party/WebKit/Source/core/core.gypi
[add] https://crrev.com/6b045cf7e0fe8f7eb94dc94508d75dbde7e63365/third_party/WebKit/Source/core/css/CSSPendingSubstitutionValue.cpp
[add] https://crrev.com/6b045cf7e0fe8f7eb94dc94508d75dbde7e63365/third_party/WebKit/Source/core/css/CSSPendingSubstitutionValue.h
[modify] https://crrev.com/6b045cf7e0fe8f7eb94dc94508d75dbde7e63365/third_party/WebKit/Source/core/css/CSSValue.cpp
[modify] https://crrev.com/6b045cf7e0fe8f7eb94dc94508d75dbde7e63365/third_party/WebKit/Source/core/css/CSSValue.h
[modify] https://crrev.com/6b045cf7e0fe8f7eb94dc94508d75dbde7e63365/third_party/WebKit/Source/core/css/StylePropertySerializer.cpp
[modify] https://crrev.com/6b045cf7e0fe8f7eb94dc94508d75dbde7e63365/third_party/WebKit/Source/core/css/parser/CSSPropertyParser.cpp
[modify] https://crrev.com/6b045cf7e0fe8f7eb94dc94508d75dbde7e63365/third_party/WebKit/Source/core/css/resolver/CSSVariableResolver.cpp
[modify] https://crrev.com/6b045cf7e0fe8f7eb94dc94508d75dbde7e63365/third_party/WebKit/Source/core/css/resolver/CSSVariableResolver.h
[modify] https://crrev.com/6b045cf7e0fe8f7eb94dc94508d75dbde7e63365/third_party/WebKit/Source/core/css/resolver/StyleBuilderCustom.cpp
[modify] https://crrev.com/6b045cf7e0fe8f7eb94dc94508d75dbde7e63365/third_party/WebKit/Source/core/css/resolver/StyleResolverState.cpp
[modify] https://crrev.com/6b045cf7e0fe8f7eb94dc94508d75dbde7e63365/third_party/WebKit/Source/core/css/resolver/StyleResolverState.h