Revert of Revert of Implement unset value handling

Source/core/css/CSSValue.cpp

Index: Source/core/css/CSSValue.cpp
diff --git a/Source/core/css/CSSValue.cpp b/Source/core/css/CSSValue.cpp
index 1c7fbd839061016f0456c026cc26538e05532317..2d55581c4cb0d99132da46a10e080bea7ac4e220 100644
--- a/Source/core/css/CSSValue.cpp
+++ b/Source/core/css/CSSValue.cpp
@@ -53,6 +53,7 @@
 #include "core/css/CSSTimingFunctionValue.h"
 #include "core/css/CSSTransformValue.h"
 #include "core/css/CSSUnicodeRangeValue.h"
+#include "core/css/CSSUnsetValue.h"
 #include "core/css/CSSValueList.h"
 
 namespace blink {
@@ -121,6 +122,8 @@
             return compareCSSValues<CSSInheritedValue>(*this, other);
         case InitialClass:
             return compareCSSValues<CSSInitialValue>(*this, other);
+        case UnsetClass:
+            return compareCSSValues<CSSUnsetValue>(*this, other);
         case GridLineNamesClass:
             return compareCSSValues<CSSGridLineNamesValue>(*this, other);
         case GridTemplateAreasClass:
@@ -191,6 +194,8 @@
         return toCSSImageValue(this)->customCSSText();
     case InheritedClass:
         return toCSSInheritedValue(this)->customCSSText();
+    case UnsetClass:
+        return toCSSUnsetValue(this)->customCSSText();
     case InitialClass:
         return toCSSInitialValue(this)->customCSSText();
     case GridLineNamesClass:
@@ -272,6 +277,9 @@
     case InitialClass:
         delete toCSSInitialValue(this);
         return;
+    case UnsetClass:
+        delete toCSSUnsetValue(this);
+        return;
     case GridLineNamesClass:
         delete toCSSGridLineNamesValue(this);
         return;
@@ -366,6 +374,9 @@
     case InitialClass:
         toCSSInitialValue(this)->~CSSInitialValue();
         return;
+    case UnsetClass:
+        toCSSUnsetValue(this)->~CSSUnsetValue();
+        return;
     case GridLineNamesClass:
         toCSSGridLineNamesValue(this)->~CSSGridLineNamesValue();
         return;
@@ -460,6 +471,9 @@
     case InitialClass:
         toCSSInitialValue(this)->traceAfterDispatch(visitor);
         return;
+    case UnsetClass:
+        toCSSUnsetValue(this)->traceAfterDispatch(visitor);
+        return;
     case GridLineNamesClass:
         toCSSGridLineNamesValue(this)->traceAfterDispatch(visitor);
         return;