Revert r154797: "Move isValid/isCurrentColor from Color to StyleColor"

Source/core/css/CSSGradientValue.h

Index: Source/core/css/CSSGradientValue.h
diff --git a/Source/core/css/CSSGradientValue.h b/Source/core/css/CSSGradientValue.h
index 2eed10b981fca8325909b1e00bbb968bf649d65a..75e51520a94fa585304e913c64213d77d19be1c4 100644
--- a/Source/core/css/CSSGradientValue.h
+++ b/Source/core/css/CSSGradientValue.h
@@ -28,7 +28,6 @@
 
 #include "core/css/CSSImageGeneratorValue.h"
 #include "core/css/CSSPrimitiveValue.h"
-#include "core/css/StyleColor.h"
 #include "wtf/RefPtr.h"
 #include "wtf/Vector.h"
 
@@ -52,7 +51,7 @@ struct CSSGradientColorStop {
     CSSGradientColorStop() : m_colorIsDerivedFromElement(false) { };
     RefPtr<CSSPrimitiveValue> m_position; // percentage or length
     RefPtr<CSSPrimitiveValue> m_color;
-    StyleColor m_resolvedColor;
+    Color m_resolvedColor;
     bool m_colorIsDerivedFromElement;
     bool operator==(const CSSGradientColorStop& other) const
     {
@@ -90,7 +89,7 @@ public:
     bool knownToBeOpaque(const RenderObject*) const;
 
     void loadSubimages(ResourceFetcher*) { }
-    PassRefPtr<CSSGradientValue> gradientWithStylesResolved(const TextLinkColors&);
+    PassRefPtr<CSSGradientValue> gradientWithStylesResolved(const TextLinkColors&, Color currentColor);
 
 protected:
     CSSGradientValue(ClassType classType, CSSGradientRepeat repeat, CSSGradientType gradientType)