IME for Mus: Make InputMethodMus use the IME Mojo API.

services/ui/public/interfaces/ime.mojom

Index: services/ui/public/interfaces/ime.mojom
diff --git a/services/ui/public/interfaces/ime.mojom b/services/ui/public/interfaces/ime.mojom
index ed1f56d15a26f4ebd202b8cd677a2bb7c3abda53..17d91070c30454fac083725453fa53e9d2f9559c 100644
--- a/services/ui/public/interfaces/ime.mojom
+++ b/services/ui/public/interfaces/ime.mojom
@@ -93,15 +93,24 @@ interface InputMethod {
   // Mus translates it to global coordinates and sends it to IME app.
   OnCaretBoundsChanged(gfx.mojom.Rect caret_bounds);
 
+  // Called to process a key event. If InputMethod handles the event, it will
+  // notify the client of the resulting composition events (there might be none)
+  // by calling TextInputClient::OnCompositionEvent(). If the event is not
+  // handled by InputMethod, TextInputClient::OnUnhandledEvent() is called.
   ProcessKeyEvent(ui.mojom.Event key_event);
+
   CancelComposition();
 };
 
 // IME drivers send updates to clients using the TextInputClient interface.
 interface TextInputClient {
   // Corresponds to "input method result" functions of ui::TextInputClient.
+  // See comments for InputMethod::ProcessKeyEvent() for when this is called.
   OnCompositionEvent(CompositionEvent event);
 
+  // See comments for InputMethod::ProcessKeyEvent() for when this is called.
+  OnUnhandledEvent(ui.mojom.Event key_event);
+
   // TODO(moshayedi): Add functions corresponding to ui::TextInputClient for:
   // - Input text confirmation
   // - Document content operations