[DesktopCapturer] FallbackDesktopCapturerWrapper and its tests

webrtc/modules/desktop_capture/fake_desktop_capturer.h

 /*
  *  Copyright (c) 2016 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.
  */
 
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   FakeDesktopCapturer();
   ~FakeDesktopCapturer() override;
 
   // Decides the result which will be returned in next Capture() callback.
   void set_result(DesktopCapturer::Result result);
 
   // Uses the |generator| provided as DesktopFrameGenerator, FakeDesktopCapturer
   // does not take the ownership of |generator|.
   void set_frame_generator(DesktopFrameGenerator* generator);
 
+  // Count of DesktopFrame(s) have been returned by this instance. This field
+  // would never be negative.
+  int num_frames_captured() const;
+
+  // Count of CaptureFrame() calls have been made. This field would never be
+  // negative.
+  int num_capture_attempts() const;
+
   // DesktopCapturer interface
   void Start(DesktopCapturer::Callback* callback) override;
   void CaptureFrame() override;
   void SetSharedMemoryFactory(
       std::unique_ptr<SharedMemoryFactory> shared_memory_factory) override;
   bool GetSourceList(DesktopCapturer::SourceList* sources) override;
   bool SelectSource(DesktopCapturer::SourceId id) override;
 
  private:
   static constexpr DesktopCapturer::SourceId kWindowId = 1378277495;
   static constexpr DesktopCapturer::SourceId kScreenId = 1378277496;
 
-  DesktopCapturer::Callback* callback_;
+  DesktopCapturer::Callback* callback_ = nullptr;
   std::unique_ptr<SharedMemoryFactory> shared_memory_factory_;
-  DesktopCapturer::Result result_;
-  DesktopFrameGenerator* generator_;
+  DesktopCapturer::Result result_ = Result::SUCCESS;
+  DesktopFrameGenerator* generator_ = nullptr;
+  int num_frames_captured_ = 0;
+  int num_capture_attempts_ = 0;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_DESKTOP_CAPTURE_FAKE_DESKTOP_CAPTURER_H_