[WebRTC] Add ScreenCapturerDifferWrapper to share Differ across ScreenCapturers

webrtc/modules/desktop_capture/fake_screen_capturer.h

Index: webrtc/modules/desktop_capture/fake_screen_capturer.h
diff --git a/webrtc/modules/desktop_capture/fake_screen_capturer.h b/webrtc/modules/desktop_capture/fake_screen_capturer.h
new file mode 100644
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+/*
+ *  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.
+ */
+
+#ifndef WEBRTC_MODULES_DESKTOP_CAPTURE_FAKE_SCREEN_CAPTURER_H_
+#define WEBRTC_MODULES_DESKTOP_CAPTURE_FAKE_SCREEN_CAPTURER_H_
+
+#include "webrtc/modules/desktop_capture/fake_desktop_capturer.h"
+#include "webrtc/modules/desktop_capture/screen_capturer.h"
+
+namespace webrtc {
+
+class FakeScreenCapturer : public FakeDesktopCapturer<ScreenCapturer> {
+ public:
+  FakeScreenCapturer();
+  ~FakeScreenCapturer() override;
+
+  // ScreenCapturer interface.
+  bool GetScreenList(ScreenList* list) override;
+  bool SelectScreen(ScreenId id) override;
+
+ private:
+  // A random ScreenId.
+  const ScreenId id_;
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_DESKTOP_CAPTURE_FAKE_SCREEN_CAPTURER_H_