Mark POWER key as system key so that it won't be passed to regular apps.

ash/accelerators/accelerator_delegate.h

Index: ash/accelerators/accelerator_delegate.h
diff --git a/ash/accelerators/accelerator_delegate.h b/ash/accelerators/accelerator_delegate.h
index 778210d0c4c5e6829aa49e5c2e21b0719d2f6190..0614f873d9d8392c2ec9389c666d10cfbd8be0f8 100644
--- a/ash/accelerators/accelerator_delegate.h
+++ b/ash/accelerators/accelerator_delegate.h
@@ -20,8 +20,7 @@ class ASH_EXPORT AcceleratorDelegate
 
   // wm::AcceleratorDelegate:
   bool ProcessAccelerator(const ui::KeyEvent& event,
-                          const ui::Accelerator& accelerator,
-                          KeyType key_type) override;
+                          const ui::Accelerator& accelerator) override;
 
  private:
   // Returns true if the window should be allowed a chance to handle