Added implementation of SyncEventObserver to route events to Javascript Extension.

chrome/common/extensions/api/sync_file_system.idl

Index: chrome/common/extensions/api/sync_file_system.idl
diff --git a/chrome/common/extensions/api/sync_file_system.idl b/chrome/common/extensions/api/sync_file_system.idl
index 5971efe9dedaa2760ee32ba98bcfa63f03ff5e0d..d6487d87c90ae6378da01dbb2294a2e8a10b5bf0 100644
--- a/chrome/common/extensions/api/sync_file_system.idl
+++ b/chrome/common/extensions/api/sync_file_system.idl
@@ -14,6 +14,12 @@ namespace syncFileSystem {
     DOMString description;
   };
   
+  enum SyncOperationType {
+    FILE_ADDED,
+    FILE_UPDATED,
+    FILE_DELETED

[kinuko, 2012/11/28 11:06:40]

I was looking at other idl files that use enum and noticed that (at least for
input side) most of them use lower-capital names (like 'tcp, 'udp' for socket
type) and in Javascript layer they're actually recognized as string enum.

Maybe to follow the convention we should also use lower-case enum values like: {
add, update, delete } ?

(And if we do so we should also do same for SyncStateStatus)

[calvinlo, 2012/11/28 12:26:34]

Done.

+  };
+  
   enum SyncStateStatus {
     // The sync service is being initialized (e.g. restoring data from the
     // database, checking connectivity and authenticating to the service etc).
@@ -71,5 +77,10 @@ namespace syncFileSystem {
     // sync backend.  (E.g. the sync is temporarily disabled due to
     // network or authentication error etc)
     static void onSyncStateChanged(SyncState detail);
+    
+    // Fired when a file has been updated by the background sync service.
+    // TODO(calvinlo): Convert |file_entry_path| from to Webkit FileEntry.
+    static void onFileSynced(DOMString file_entry_path,
+                             SyncOperationType operation);
   };
 };