Re-land "Convert native handles to buffers before encoding."

webrtc/common_video/i420_buffer_pool.cc

Index: webrtc/common_video/i420_buffer_pool.cc
diff --git a/webrtc/common_video/i420_buffer_pool.cc b/webrtc/common_video/i420_buffer_pool.cc
index 35a6c109539b56f5832a398f5679dc0d7d7ae95d..4b8695824c794ef50f402b4cc2a22e06cc73ded3 100644
--- a/webrtc/common_video/i420_buffer_pool.cc
+++ b/webrtc/common_video/i420_buffer_pool.cc
@@ -40,6 +40,11 @@ class PooledI420Buffer : public webrtc::VideoFrameBuffer {
   }
   void* native_handle() const override { return nullptr; }
 
+  rtc::scoped_refptr<VideoFrameBuffer> NativeToI420Buffer() override {
+    RTC_NOTREACHED();
+    return nullptr;
+  }
+
   friend class rtc::RefCountedObject<PooledI420Buffer>;
   rtc::scoped_refptr<webrtc::I420Buffer> buffer_;
 };