Adds CaptureClientObserver as a way to track changes in capture

ash/display/mirror_window_controller.cc

 // Copyright (c) 2013 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.
 
 #include "ash/display/mirror_window_controller.h"
 
 #include <utility>
 
 #if defined(USE_X11)
 #include <X11/extensions/XInput2.h>
@@ skipping 76 matching lines @@
                  const display::Display& display) override {
     NOTREACHED();
   }
 
  private:
   MirrorWindowController* controller_;  // not owned.
 
   DISALLOW_COPY_AND_ASSIGN(MirroringScreenPositionClient);
 };
 
+// A trivial CaptureClient that does nothing. That is, calls to set/release
+// capture are dropped.
 class NoneCaptureClient : public aura::client::CaptureClient {
  public:
   NoneCaptureClient() {}
   ~NoneCaptureClient() override {}
 
  private:
-  // Does a capture on the |window|.
+  // aura::client::CaptureClient:
   void SetCapture(aura::Window* window) override {}
-
-  // Releases a capture from the |window|.
   void ReleaseCapture(aura::Window* window) override {}
-
-  // Returns the current capture window.
-  aura::Window* GetCaptureWindow() override { return NULL; }
-  aura::Window* GetGlobalCaptureWindow() override { return NULL; }
+  aura::Window* GetCaptureWindow() override { return nullptr; }
+  aura::Window* GetGlobalCaptureWindow() override { return nullptr; }
+  void AddObserver(aura::client::CaptureClientObserver* observer) override {}
+  void RemoveObserver(aura::client::CaptureClientObserver* observer) override {}
 
   DISALLOW_COPY_AND_ASSIGN(NoneCaptureClient);
 };
 
 DisplayManager::MultiDisplayMode GetCurrentMultiDisplayMode() {
   DisplayManager* display_manager = Shell::GetInstance()->display_manager();
   return display_manager->IsInUnifiedMode()
              ? DisplayManager::UNIFIED
              : (display_manager->IsInMirrorMode() ? DisplayManager::MIRRORING
                                                   : DisplayManager::EXTENDED);
@@ skipping 243 matching lines @@
   // EventProcessor may be accessed after this call if the mirroring window
   // was deleted as a result of input event (e.g. shortcut), so don't delete
   // now.
   if (delay_host_deletion)
     base::ThreadTaskRunnerHandle::Get()->DeleteSoon(FROM_HERE, host_info);
   else
     delete host_info;
 }
 
 }  // namespace ash