Fix WebrtcVideoStream to handle failed capture requests.

remoting/codec/webrtc_video_encoder.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef REMOTING_CODEC_WEBRTC_VIDEO_ENCODER_H_
 #define REMOTING_CODEC_WEBRTC_VIDEO_ENCODER_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 35 matching lines @@
 
   struct EncodedFrame {
     webrtc::DesktopSize size;
     std::string data;
     bool key_frame;
     int quantizer;
   };
 
   virtual ~WebrtcVideoEncoder() {}
 
-  // Encode an image stored in |frame|. If |frame.updated_region()| is empty
-  // then the encoder may return a packet (e.g. to top-off previously-encoded
+  // Encode an image stored in |frame|. If frame.updated_region() is empty
+  // then the encoder may return a frame (e.g. to top-off previously-encoded
   // portions of the frame to higher quality) or return nullptr to indicate that
-  // there is no work to do.
+  // there is no work to do. |frame| may be nullptr. This case must be handled
+  // the same as if frame.updated_region() is empty.
   virtual std::unique_ptr<EncodedFrame> Encode(
-      const webrtc::DesktopFrame& frame,
+      const webrtc::DesktopFrame* frame,
       const FrameParams& param) = 0;
 };
 
 }  // namespace remoting
 
 #endif  // REMOTING_CODEC_WEBRTC_VIDEO_ENCODER_H_