Makes NativeViewport send OnBoundsChanged() after widget available

mojo/services/public/interfaces/native_viewport/native_viewport.mojom

Index: mojo/services/public/interfaces/native_viewport/native_viewport.mojom
diff --git a/mojo/services/public/interfaces/native_viewport/native_viewport.mojom b/mojo/services/public/interfaces/native_viewport/native_viewport.mojom
index 0a685328f72020ecd583c5e3a0cb5d387dd84e27..6f05b7243dd0665dff3f93b4c28db2db97ebd8ac 100644
--- a/mojo/services/public/interfaces/native_viewport/native_viewport.mojom
+++ b/mojo/services/public/interfaces/native_viewport/native_viewport.mojom
@@ -13,7 +13,6 @@ module mojo {
 interface NativeViewport {
   // TODO(sky): having a create function is awkward. Should there be a factory
   // to create the NativeViewport that takes the size?
-  // TODO(sky): callback should take size too.
   Create(Size size) => (uint64 native_viewport_id);
   Show();
   Hide();
@@ -23,6 +22,8 @@ interface NativeViewport {
 };
 
 interface NativeViewportClient {
+  // OnSizeChanged() is sent at least once after the callback from Create() is
+  // called.
   OnSizeChanged(Size size);
   OnDestroyed();
   OnEvent(Event event) => ();