Added checks to handle unsyncable extensions.

chrome/browser/sync/glue/extension_model_associator.h

Index: chrome/browser/sync/glue/extension_model_associator.h
diff --git a/chrome/browser/sync/glue/extension_model_associator.h b/chrome/browser/sync/glue/extension_model_associator.h
index d88be10f7fe808c873848e06d5e9b2e23f86a8f7..23ffd37eaea7d46ce63682653b0fd27fabbae2bb 100644
--- a/chrome/browser/sync/glue/extension_model_associator.h
+++ b/chrome/browser/sync/glue/extension_model_associator.h
@@ -55,17 +55,17 @@ class ExtensionModelAssociator : public AssociatorInterface {
 
   // TODO(akalin): Return an error string instead of just a bool.
   bool OnClientUpdate(const std::string& id);
-  void OnServerUpdate(const sync_pb::ExtensionSpecifics& server_data);
-  void OnServerRemove(const std::string& id);
+
+  // |extension| may be NULL (if the client does not have the
+  // extension installed).
+  void OnServerUpdate(const sync_pb::ExtensionSpecifics& server_data,
+                      Extension* extension);
 
  private:
   // Returns the extension service from |sync_service_|.  Never
   // returns NULL.
   ExtensionsService* GetExtensionsService();
 
-  bool GetExtensionDataFromClient(
-      const std::string& id, sync_pb::ExtensionSpecifics* client_data);
-
   bool GetExtensionDataFromServer(
       const std::string& id, sync_api::WriteTransaction* trans,
       const sync_api::ReadNode& root,