[Sync] Refactor sync datatype error handling.

chrome/browser/sync/api/sync_change_processor.h

Index: chrome/browser/sync/api/sync_change_processor.h
diff --git a/chrome/browser/sync/api/sync_change_processor.h b/chrome/browser/sync/api/sync_change_processor.h
index 80be610ab5b4902bf0e1f6021108af4f1f38f2e2..edc7c0faa55a4da9d4ad65ff4fdad692a5596f8c 100644
--- a/chrome/browser/sync/api/sync_change_processor.h
+++ b/chrome/browser/sync/api/sync_change_processor.h
@@ -8,6 +8,8 @@
 
 #include <vector>
 
+#include "chrome/browser/sync/api/sync_error.h"
+
 class SyncChange;
 
 namespace tracked_objects {
@@ -20,8 +22,15 @@ typedef std::vector<SyncChange> SyncChangeList;
 class SyncChangeProcessor {
  public:
   // Process a list of SyncChanges.
-  virtual void ProcessSyncChanges(const tracked_objects::Location& from_here,
-                                  const SyncChangeList& change_list) = 0;
+  // Returns: A default SyncError (IsSet() == false) if no errors were
+  //          encountered, and a filled SyncError (IsSet() == true)
+  //          otherwise.
+  // Inputs:
+  //   |from_here|: allows tracking of where sync changes originate.
+  //   |change_list|: is the list of sync changes in need of processing.
+  virtual SyncError ProcessSyncChanges(
+      const tracked_objects::Location& from_here,
+      const SyncChangeList& change_list) = 0;
  protected:
   virtual ~SyncChangeProcessor();
 };