[DesktopCapturer] FallbackDesktopCapturerWrapper and its tests

webrtc/modules/desktop_capture/shared_memory.h

Index: webrtc/modules/desktop_capture/shared_memory.h
diff --git a/webrtc/modules/desktop_capture/shared_memory.h b/webrtc/modules/desktop_capture/shared_memory.h
index 6e15f23f6b8a4a0429f2d4aa2c09228f447d03bc..df4a91973e66fab65357ea71d115162c991070ba 100644
--- a/webrtc/modules/desktop_capture/shared_memory.h
+++ b/webrtc/modules/desktop_capture/shared_memory.h
@@ -76,6 +76,24 @@ class SharedMemoryFactory {
   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_