[Sync] Add comment for SyncBackendHost::OnBackendInitialized

BUG=
TEST=

Review URL: http://codereview.chromium.org/7614001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@96587 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/sync/engine/syncapi.h

@@ -845,7 +845,8 @@ class SyncManager {
     // WARNING: Calling methods on the SyncManager before receiving this
     // message, unless otherwise specified, produces undefined behavior.
     //
-    // The given backend can emit the following events:
+    // |js_backend| is what about:sync interacts with.  It can emit
+    // the following events:
 
     /**
      * @param {{ enabled: boolean }} details A dictionary containing:
@@ -861,8 +862,8 @@ class SyncManager {
      */
     // function onIncomingNotification(details);
 
-    // The given backend responds to the following messages (all other
-    // messages are ignored):
+    // Also, it responds to the following messages (all other messages
+    // are ignored):
 
     /**
      * Gets the current notification state.

chrome/browser/sync/glue/sync_backend_host.h

@@ -55,9 +55,13 @@ 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;