Add the basic code skeleton for system info event router

chrome/common/extensions/api/experimental_system_info_storage.idl

Index: chrome/common/extensions/api/experimental_system_info_storage.idl
diff --git a/chrome/common/extensions/api/experimental_system_info_storage.idl b/chrome/common/extensions/api/experimental_system_info_storage.idl
index d43116312af61fc56d5ccd142a896fbac375b24e..13ed295ad85680c962d6bbd15ee947ba84792429 100644
--- a/chrome/common/extensions/api/experimental_system_info_storage.idl
+++ b/chrome/common/extensions/api/experimental_system_info_storage.idl
@@ -25,11 +25,26 @@ namespace experimental.systemInfo.storage {
     double availableCapacity;
   };
 
+  dictionary StorageChangeInfo {
+    // The uniue id of the storage unit already changed.
+    DOMString id;
+    // The new value of the available capacity.
+    double availableCapacity;
+  };
+
   callback StorageInfoCallback = void (StorageUnitInfo[] info);
 
   interface Functions {
-    // Get the storage information from the system.
+    // Get the storage information from the system. The argument passed to the
+    // callback is an array of StorageUnitInfo objects.
     static void get(StorageInfoCallback callback);
   };
+
+  interface Events {
+    // Fired when the storage information is updated.
+    // |info|   : The changed information for the specified storage unit
+    static void onAvailableCapacityChanged(StorageChangeInfo info);
+  };
+ 
 };