Do not activate system tray bubble by default

ash/mus/bridge/wm_shell_mus.cc

Index: ash/mus/bridge/wm_shell_mus.cc
diff --git a/ash/mus/bridge/wm_shell_mus.cc b/ash/mus/bridge/wm_shell_mus.cc
index 7c720a87d6ad8e634e968596c9a8b9bbe164bea0..d8fbf27be09f9f57ee944df6af55c80e3a5355fd 100644
--- a/ash/mus/bridge/wm_shell_mus.cc
+++ b/ash/mus/bridge/wm_shell_mus.cc
@@ -7,6 +7,7 @@
 #include <utility>
 
 #include "ash/common/accelerators/accelerator_controller.h"
+#include "ash/common/key_event_watcher.h"
 #include "ash/common/session/session_state_delegate.h"
 #include "ash/common/shell_delegate.h"
 #include "ash/common/shell_observer.h"
@@ -364,6 +365,12 @@ WmShellMus::CreateImmersiveFullscreenController() {
   return base::MakeUnique<ImmersiveFullscreenController>();
 }
 
+std::unique_ptr<KeyEventWatcher> WmShellMus::CreateKeyEventWatcher() {
+  // TODO: needs implementation for mus, http://crbug.com/649600.
+  NOTIMPLEMENTED();
+  return std::unique_ptr<KeyEventWatcher>();
+}
+
 void WmShellMus::OnOverviewModeStarting() {
   FOR_EACH_OBSERVER(ShellObserver, *shell_observers(),
                     OnOverviewModeStarting());