Adds ability for pre-target accelerators to not consume events

ash/mus/window_manager.cc

Index: ash/mus/window_manager.cc
diff --git a/ash/mus/window_manager.cc b/ash/mus/window_manager.cc
index 3f11af23b86cb72b530d135b27f477c807baf898..36582f6fe188272634b6a3900b7875880d0cf804 100644
--- a/ash/mus/window_manager.cc
+++ b/ash/mus/window_manager.cc
@@ -229,7 +229,8 @@ void WindowManager::OnWmNewDisplay(::ui::Window* window,
   CreateRootWindowController(window, display);
 }
 
-void WindowManager::OnAccelerator(uint32_t id, const ui::Event& event) {
+ui::mojom::EventResult WindowManager::OnAccelerator(uint32_t id,
+                                                    const ui::Event& event) {
   switch (id) {
     case kWindowSwitchAccelerator:
       window_manager_client()->ActivateNextWindow();
@@ -239,6 +240,7 @@ void WindowManager::OnAccelerator(uint32_t id, const ui::Event& event) {
                         OnAccelerator(id, event));
       break;
   }
+  return ui::mojom::EventResult::HANDLED;
 }
 
 }  // namespace mus