Allow EmbedRoot to be called multiple times.

mojo/services/public/interfaces/view_manager/view_manager.mojom

Index: mojo/services/public/interfaces/view_manager/view_manager.mojom
diff --git a/mojo/services/public/interfaces/view_manager/view_manager.mojom b/mojo/services/public/interfaces/view_manager/view_manager.mojom
index caf57a952e301ea8e851f35c0bc759ce2da62e1f..5edd9cee24893febf080314fce57ad195253be96 100644
--- a/mojo/services/public/interfaces/view_manager/view_manager.mojom
+++ b/mojo/services/public/interfaces/view_manager/view_manager.mojom
@@ -22,10 +22,17 @@ enum ErrorCode {
   ILLEGAL_ARGUMENT,
 };
 
-// ViewManagerInitService is responsible for launching the client that controls
-// the root node. mojo::view_manager returns an instance of this. All other
-// connections are established by the client this creates.
+// ViewManagerInitService is used to grant an application a connection to the
+// ViewManager by embedding it at an approriate Node.
 interface ViewManagerInitService {
+  // Embed the application @ |url| at an appropriate Node.
+  // The first time this method is called in the lifetime of a View Manager
+  // application instance, the "appropriate Node" is defined as being the
+  // service root Node.
+  // Subsequent times, implementation of this method is delegated to the
+  // application embedded at the service root Node. This application is
+  // typically referred to as the "window manager", and will have a specific
+  // definition of where within its Node hierarchy to embed an unparented URL.
   EmbedRoot(string url) => (bool success);
 };
 
@@ -193,8 +200,16 @@ interface ViewManagerClient {
   //             removed.
   OnFocusChanged(uint32 gained_focus_id, uint32 lost_focus_id);
 
-  // TODO(sky): move to separate interface when FIFO sorted out.
+  // TODO(sky): The following methods represent an interface between the view
+  //            manager and the application embedded at the service root node
+  //            (i.e. the window manager). These methods are not called on
+  //            any other clients. They should be moved to a separate interface
+  //            once support for derived FIFOs is landed.
 
+  // Requests the window manager create a "top level" node embedding |url|.
+  EmbedRoot(string url);
+
+  // Requests the view manager dispatch the event targeted at |view|.
   DispatchOnViewInputEvent(uint32 view, mojo.Event event);
 };