[DesktopCapturer] FallbackDesktopCapturerWrapper and its tests

webrtc/modules/desktop_capture/shared_memory.h

 /*
  *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 58 matching lines @@
  public:
   SharedMemoryFactory() {}
   virtual ~SharedMemoryFactory() {}
 
   virtual std::unique_ptr<SharedMemory> CreateSharedMemory(size_t size) = 0;
 
  private:
   RTC_DISALLOW_COPY_AND_ASSIGN(SharedMemoryFactory);
 };
 
+// Implementation to share a SharedMemoryFactory between DesktopCapturer
+// instances.
+class SharedMemoryFactoryWrapper : public SharedMemoryFactory {
+ public:
+  ~SharedMemoryFactoryWrapper() override;
+
+  // |factory| must not be nullptr, and it must overlive this instance.
+  static std::unique_ptr<SharedMemoryFactory> Wrap(
+      const std::unique_ptr<SharedMemoryFactory>& factory);
+
+  std::unique_ptr<SharedMemory> CreateSharedMemory(size_t size) override;
+
+ private:
+  SharedMemoryFactoryWrapper(SharedMemoryFactory* factory);
+
+  SharedMemoryFactory* factory_;
+};
+
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_DESKTOP_CAPTURE_SHARED_MEMORY_H_