[Variables] Enable get/setProperty and similar APIs from the CSSOM

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

Index: third_party/WebKit/Source/core/css/CSSValue.cpp
diff --git a/third_party/WebKit/Source/core/css/CSSValue.cpp b/third_party/WebKit/Source/core/css/CSSValue.cpp
index dcc6bd14b2331aeddf7a30a78caa19cb5f9f8d11..462a56035770413d8a5d1fe2671805dae09e1770 100644
--- a/third_party/WebKit/Source/core/css/CSSValue.cpp
+++ b/third_party/WebKit/Source/core/css/CSSValue.cpp
@@ -261,8 +261,7 @@ String CSSValue::cssText() const
     case VariableReferenceClass:
         return toCSSVariableReferenceValue(this)->customCSSText();
     case CustomPropertyDeclarationClass:
-        // TODO(leviw): We don't allow custom properties in CSSOM yet
-        ASSERT_NOT_REACHED();
+        return toCSSCustomPropertyDeclaration(this)->customCSSText();
     }
     ASSERT_NOT_REACHED();
     return String();