Revert of Remove InputMethodEventHandler.

ash/mus/ash_window_tree_host_mus.cc

Index: ash/mus/ash_window_tree_host_mus.cc
diff --git a/ash/mus/ash_window_tree_host_mus.cc b/ash/mus/ash_window_tree_host_mus.cc
index b5c2deaad92134708631acc2ca13a74c68a0d57d..7c10818a2cb53502302e5b63d78e7964aa72d2ac 100644
--- a/ash/mus/ash_window_tree_host_mus.cc
+++ b/ash/mus/ash_window_tree_host_mus.cc
@@ -6,6 +6,7 @@
 
 #include "ash/host/root_window_transformer.h"
 #include "ash/host/transformer_helper.h"
+#include "ash/ime/input_method_event_handler.h"
 #include "base/memory/ptr_util.h"
 #include "ui/aura/mus/window_tree_host_mus_init_params.h"
 #include "ui/aura/window.h"
@@ -85,4 +86,15 @@ void AshWindowTreeHostMus::UpdateRootWindowSizeInPixels(
   transformer_helper_->UpdateWindowSize(host_size_in_pixels);
 }
 
+ui::EventDispatchDetails AshWindowTreeHostMus::DispatchKeyEventPostIME(
+    ui::KeyEvent* event) {
+  // input_method_handler() can be null when using IME service with --mus.
+  if (input_method_handler())
+    input_method_handler()->SetPostIME(true);
+  ui::EventDispatchDetails details = SendEventToSink(event);
+  if (input_method_handler() && !details.dispatcher_destroyed)
+    input_method_handler()->SetPostIME(false);
+  return details;
+}
+
 }  // namespace ash