Sync/Valgrind: Add gmock printers for SyncChange, SyncData, SyncError.

chrome/browser/sync/api/sync_error.h

Index: chrome/browser/sync/api/sync_error.h
diff --git a/chrome/browser/sync/api/sync_error.h b/chrome/browser/sync/api/sync_error.h
index c4bc7c3079c8a10d0932fc9276a4d97640ba81b8..924495c2177cc900c4cb7f9de0b85b5cae7554a3 100644
--- a/chrome/browser/sync/api/sync_error.h
+++ b/chrome/browser/sync/api/sync_error.h
@@ -6,6 +6,7 @@
 #define CHROME_BROWSER_SYNC_API_SYNC_ERROR_H_
 #pragma once
 
+#include <iosfwd>
 #include <string>
 
 #include "base/memory/scoped_ptr.h"
@@ -76,4 +77,7 @@ class SyncError {
   syncable::ModelType type_;
 };
 
+// Printing operator - helps gtest produce readable error messages.
+std::ostream& operator<<(std::ostream& os, const SyncError& sync_error);
+
 #endif  // CHROME_BROWSER_SYNC_API_SYNC_ERROR_H_