[Sync] Add comment for SyncBackendHost::OnBackendInitialized

chrome/browser/sync/glue/sync_backend_host.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_SYNC_GLUE_SYNC_BACKEND_HOST_H_
 #define CHROME_BROWSER_SYNC_GLUE_SYNC_BACKEND_HOST_H_
 #pragma once
 
 #include <map>
 #include <string>
@@ skipping 37 matching lines @@
 
 // SyncFrontend is the interface used by SyncBackendHost to communicate with
 // the entity that created it and, presumably, is interested in sync-related
 // activity.
 // NOTE: All methods will be invoked by a SyncBackendHost on the same thread
 // used to create that SyncBackendHost.
 class SyncFrontend {
  public:
   SyncFrontend() {}
 
-  // The backend has completed initialization and it is now ready to accept and
-  // process changes.  If success is false, initialization wasn't able to be
-  // completed and should be retried.
+  // The backend has completed initialization and it is now ready to
+  // accept and process changes.  If success is false, initialization
+  // wasn't able to be completed and should be retried.
+  //
+  // |js_backend| is what about:sync interacts with; it's different
+  // from the 'Backend' in 'OnBackendInitialized' (unfortunately).  It
+  // is initialized only if |success| is true.
   virtual void OnBackendInitialized(
       const WeakHandle<JsBackend>& js_backend, bool success) = 0;
 
   // The backend queried the server recently and received some updates.
   virtual void OnSyncCycleCompleted() = 0;
 
   // The backend encountered an authentication problem and requests new
   // credentials to be provided. See SyncBackendHost::Authenticate for details.
   virtual void OnAuthError() = 0;
 
@@ skipping 535 matching lines @@
 
   // UI-thread cache of the last SyncSessionSnapshot received from syncapi.
   scoped_ptr<sessions::SyncSessionSnapshot> last_snapshot_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncBackendHost);
 };
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_GLUE_SYNC_BACKEND_HOST_H_