Compute initial values for registered custom properties.

third_party/WebKit/Source/core/css/PropertyRegistration.cpp

Index: third_party/WebKit/Source/core/css/PropertyRegistration.cpp
diff --git a/third_party/WebKit/Source/core/css/PropertyRegistration.cpp b/third_party/WebKit/Source/core/css/PropertyRegistration.cpp
index 410ae2c900bf7b26544e15ce2d03ed907961020c..59ca5e39c53dd9d5d5bbeef09104556c19c39d30 100644
--- a/third_party/WebKit/Source/core/css/PropertyRegistration.cpp
+++ b/third_party/WebKit/Source/core/css/PropertyRegistration.cpp
@@ -12,6 +12,7 @@
 #include "core/css/PropertyRegistry.h"
 #include "core/css/parser/CSSTokenizer.h"
 #include "core/css/parser/CSSVariableParser.h"
+#include "core/css/resolver/StyleBuilderConverter.h"
 #include "core/dom/Document.h"
 #include "core/dom/ExceptionCode.h"
 #include "core/dom/StyleChangeReason.h"
@@ -121,6 +122,8 @@ void PropertyRegistration::registerProperty(
           "The initial value provided is not computationally independent.");
       return;
     }
+    initial =
+        &StyleBuilderConverter::convertRegisteredPropertyInitialValue(*initial);
     RefPtr<CSSVariableData> initialVariableData = CSSVariableData::create(
         tokenizer.tokenRange(), isAnimationTainted, false);
     registry.registerProperty(