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..6f2e582d9ba2755d8df36b73b2bd6ff77d524783 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"
@@ -53,6 +54,8 @@ class SyncError {
   const std::string& message() const;
   syncable::ModelType type() const;
 
+  std::string ToString() const;
+
  private:
   // Print error information to log.
   void PrintLogError() const;
@@ -76,4 +79,7 @@ class SyncError {
   syncable::ModelType type_;
 };
 
+// gmock printer helper.
+void PrintTo(const SyncError& sync_error, std::ostream* os);
+
 #endif  // CHROME_BROWSER_SYNC_API_SYNC_ERROR_H_