Add paint() function as valid CSS <image> type.

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

Index: third_party/WebKit/Source/core/css/CSSValue.h
diff --git a/third_party/WebKit/Source/core/css/CSSValue.h b/third_party/WebKit/Source/core/css/CSSValue.h
index 8b52da11e1acb41fcacb1cecfa7dfd0b96221ba0..5dad727cbed805fd1f3e8dce3291db521651f372 100644
--- a/third_party/WebKit/Source/core/css/CSSValue.h
+++ b/third_party/WebKit/Source/core/css/CSSValue.h
@@ -78,6 +78,7 @@ public:
     bool isCounterValue() const { return m_classType == CounterClass; }
     bool isCursorImageValue() const { return m_classType == CursorImageClass; }
     bool isCrossfadeValue() const { return m_classType == CrossfadeClass; }
+    bool isPaintValue() const { return m_classType == PaintClass; }
     bool isFontFeatureValue() const { return m_classType == FontFeatureClass; }
     bool isFontFaceSrcValue() const { return m_classType == FontFaceSrcClass; }
     bool isFunctionValue() const { return m_classType == FunctionClass; }
@@ -148,6 +149,7 @@ protected:
 
         // Image generator classes.
         CrossfadeClass,
+        PaintClass,
         LinearGradientClass,
         RadialGradientClass,