Allow ForwardingDelegate to have a null delegate

ui/display/manager/forwarding_display_delegate.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_DISPLAY_MANAGER_FORWARDING_DISPLAY_DELEGATE_H_
 #define UI_DISPLAY_MANAGER_FORWARDING_DISPLAY_DELEGATE_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 56 matching lines @@
 
   // display::mojom::NativeDisplayObserver:
   void OnConfigurationChanged() override;
 
  private:
   // Stores display snapshots and forards them to |callback|.
   void StoreAndForwardDisplays(
       const GetDisplaysCallback& callback,
       std::vector<std::unique_ptr<DisplaySnapshotMojo>> snapshots);
 
+  // NOTE: this may be null in tests.
   mojom::NativeDisplayDelegatePtr delegate_;
   mojo::Binding<mojom::NativeDisplayObserver> binding_;
 
   // Display snapshots are owned here but accessed via raw pointers elsewhere.
   // Call OnDisplaySnapshotsInvalidated() on observers before invalidating them.
   std::vector<std::unique_ptr<DisplaySnapshotMojo>> snapshots_;
 
   base::ObserverList<display::NativeDisplayObserver> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(ForwardingDisplayDelegate);
 };
 
 }  // namespace display
 
 #endif  // UI_DISPLAY_MANAGER_FORWARDING_DISPLAY_DELEGATE_H_