Add operator<< to StyleDifference for debug logging.

third_party/WebKit/Source/core/style/StyleDifference.h

Index: third_party/WebKit/Source/core/style/StyleDifference.h
diff --git a/third_party/WebKit/Source/core/style/StyleDifference.h b/third_party/WebKit/Source/core/style/StyleDifference.h
index b7756955b884a12e4c90933757836312826af685..faafb52db1c4a7a6861f3f24694a6df06a80e7d5 100644
--- a/third_party/WebKit/Source/core/style/StyleDifference.h
+++ b/third_party/WebKit/Source/core/style/StyleDifference.h
@@ -5,6 +5,8 @@
 #ifndef StyleDifference_h
 #define StyleDifference_h
 
+#include <iosfwd>
+#include "core/CoreExport.h"
 #include "wtf/Allocator.h"
 #include "wtf/Assertions.h"
 
@@ -27,6 +29,8 @@ class StyleDifference {
     ScrollAnchorDisablingPropertyChanged = 1 << 7,
     // If you add a value here, be sure to update the number of bits on
     // m_propertySpecificDifferences.
+
+    PropertyDifferenceMax = TextDecorationOrColorChanged
   };
 
   StyleDifference()
@@ -139,6 +143,9 @@ class StyleDifference {
   }
 
  private:
+  friend CORE_EXPORT std::ostream& operator<<(std::ostream&,
+                                              const StyleDifference&);
+
   enum PaintInvalidationType {
     NoPaintInvalidation,
     PaintInvalidationObject,
@@ -153,6 +160,8 @@ class StyleDifference {
   unsigned m_propertySpecificDifferences : 8;
 };
 
+CORE_EXPORT std::ostream& operator<<(std::ostream&, const StyleDifference&);
+
 }  // namespace blink
 
 #endif  // StyleDifference_h