mus: Introduce API for embedder to dispatch event to the embeded client.

components/mus/public/interfaces/window_tree.mojom

Index: components/mus/public/interfaces/window_tree.mojom
diff --git a/components/mus/public/interfaces/window_tree.mojom b/components/mus/public/interfaces/window_tree.mojom
index 96c2f73fe75fe7e9b36912a1be8b19bbab3d5b37..5a80108d437017c905c53dbeb60cedf907be2e51 100644
--- a/components/mus/public/interfaces/window_tree.mojom
+++ b/components/mus/public/interfaces/window_tree.mojom
@@ -16,6 +16,10 @@ import "ui/events/mojo/event.mojom";
 import "ui/gfx/geometry/mojo/geometry.mojom";
 import "ui/platform_window/mojo/text_input_state.mojom";
 
+interface InputEventHandler {
+  OnInputEvent(uint32 window_id, ui.mojom.Event event) => (bool handled);
+};
+
 // Windows are identified by a uint32. The upper 16 bits are the connection id,
 // and the lower 16 the id assigned by the client.
 //
@@ -196,7 +200,7 @@ interface WindowTree {
   //
   // The callback returns whether the embedding was successful.
   Embed(uint32 window_id, WindowTreeClient client, uint32 embed_flags)
-      => (bool success);
+      => (bool success, InputEventHandler? event_handler);
 
   // Sets focus to the specified window, use 0 to clear focus. For a window to
   // get focus the following has to happen: the window is drawn, the window has
@@ -362,6 +366,8 @@ interface WindowTreeClient {
   // change ids for details.
   OnChangeCompleted(uint32 change_id, bool success);
 
+  RequestInputEventHandler(InputEventHandler& handler);
+
   // The WindowManager is requesting the specified window to close. If the
   // client allows the change it should delete the window.
   RequestClose(uint32 window_id);