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 64d62ac5b1f4a488f47af701dfcf7a86cffe2bad..b806130cb608e16953481d6e39cd16fc01a54bb9 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"
 
@@ -26,6 +28,8 @@ class StyleDifference {
     TextDecorationOrColorChanged = 1 << 6,
     // If you add a value here, be sure to update the number of bits on
     // m_propertySpecificDifferences.
+
+    PropertyDifferenceMax = TextDecorationOrColorChanged
   };
 
   StyleDifference()
@@ -139,6 +143,8 @@ class StyleDifference {
   }
 
  private:
+  friend std::ostream& operator<<(std::ostream&, const StyleDifference);
+
   enum PaintInvalidationType {
     NoPaintInvalidation,
     PaintInvalidationObject,
@@ -153,6 +159,8 @@ class StyleDifference {
   unsigned m_scrollAnchorDisablingPropertyChanged : 1;
 };
 
+CORE_EXPORT std::ostream& operator<<(std::ostream&, const StyleDifference);

[Xianzhu, 2017/03/20 21:14:11]

Nit: either 'const StyleDifference&' or 'StyleDifference'.

[wkorman, 2017/03/20 23:04:29]

Done.

+
 }  // namespace blink
 
 #endif  // StyleDifference_h