Reimplement PooledI420Buffer, without an extra indirection.

webrtc/common_video/include/i420_buffer_pool.h

Index: webrtc/common_video/include/i420_buffer_pool.h
diff --git a/webrtc/common_video/include/i420_buffer_pool.h b/webrtc/common_video/include/i420_buffer_pool.h
index 5c9ab06a097f1afbd0013cc5c6439d7b62890552..a34aa361b58f32de48a575b24814456dd84bf357 100644
--- a/webrtc/common_video/include/i420_buffer_pool.h
+++ b/webrtc/common_video/include/i420_buffer_pool.h
@@ -27,17 +27,31 @@ class I420BufferPool {
  public:
   I420BufferPool() : I420BufferPool(false) {}
   explicit I420BufferPool(bool zero_initialize);
+  ~I420BufferPool();
 
   // Returns a buffer from the pool, or creates a new buffer if no suitable
   // buffer exists in the pool.
-  rtc::scoped_refptr<VideoFrameBuffer> CreateBuffer(int width, int height);
+  rtc::scoped_refptr<I420Buffer> CreateBuffer(int width, int height);
   // Clears buffers_ and detaches the thread checker so that it can be reused
   // later from another thread.
   void Release();
 
  private:
+  class PooledI420Buffer : public webrtc::I420Buffer {
+   public:
+    PooledI420Buffer(int width, int height);
+    bool IsMutable() override;
+    bool IsFree();
+    void UnManage();
+
+   private:
+    virtual int GetRefCount() const = 0;
+    bool IsManaged() const;
+    volatile int managed_;
+  };
+
   rtc::ThreadChecker thread_checker_;
-  std::list<rtc::scoped_refptr<I420Buffer>> buffers_;
+  std::list<rtc::scoped_refptr<PooledI420Buffer>> buffers_;
   // If true, newly allocated buffers are zero-initialized. Note that recycled
   // buffers are not zero'd before reuse. This is required of buffers used by
   // FFmpeg according to http://crbug.com/390941, which only requires it for the