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

chrome/browser/sync/api/sync_data.h

Index: chrome/browser/sync/api/sync_data.h
diff --git a/chrome/browser/sync/api/sync_data.h b/chrome/browser/sync/api/sync_data.h
index b948579b610ed3bf97e26168dc99f21938ee8f22..e1c6729d8850e5c71572d8f4347d6e2a9b78d936 100644
--- a/chrome/browser/sync/api/sync_data.h
+++ b/chrome/browser/sync/api/sync_data.h
@@ -6,6 +6,7 @@
 #define CHROME_BROWSER_SYNC_API_SYNC_DATA_H_
 #pragma once
 
+#include <iosfwd>
 #include <string>
 #include <vector>
 
@@ -75,6 +76,8 @@ class SyncData {
   // Whether this sync data is for local data or data coming from the syncer.
   bool IsLocal() const;
 
+  std::string ToString() const;
+
   // TODO(zea): Query methods for other sync properties: parent, successor, etc.
 
  private:
@@ -111,4 +114,7 @@ class SyncData {
   ImmutableSyncEntity immutable_entity_;
 };
 
+// gmock printer helper.
+void PrintTo(const SyncData& sync_data, std::ostream* os);
+
 #endif  // CHROME_BROWSER_SYNC_API_SYNC_DATA_H_