[chrome.displaySource] Implement LPCM audio encoder.

extensions/renderer/api/display_source/wifi_display/wifi_display_audio_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 EXTENSIONS_RENDERER_API_DISPLAY_SOURCE_WIFI_DISPLAY_WIFI_DISPLAY_AUDIO_ENCODER_H_
 #define EXTENSIONS_RENDERER_API_DISPLAY_SOURCE_WIFI_DISPLAY_WIFI_DISPLAY_AUDIO_ENCODER_H_
 
 #include "content/public/renderer/media_stream_audio_sink.h"
 #include "extensions/renderer/api/display_source/wifi_display/wifi_display_media_encoder.h"
 #include "third_party/wds/src/libwds/public/audio_codec.h"
 
 namespace extensions {
 
 // This interface is a base class for audio used by the Wi-Fi Display media
 // pipeline.
 // Threading: the client code should belong to a single thread.
 class WiFiDisplayAudioEncoder : public WiFiDisplayMediaEncoder,
                                 public content::MediaStreamAudioSink {
  public:
   using AudioEncoderCallback =
       base::Callback<void(scoped_refptr<WiFiDisplayAudioEncoder>)>;
 
   static void Create(const wds::AudioCodec& audio_codec,
                      const AudioEncoderCallback& encoder_callback);
 
  protected:
   static const size_t kInvalidCodecModeValue = ~static_cast<size_t>(0u);
 
+  // A factory method that creates a new encoder instance for Linear Pulse-Code
+  // Modulation (LPCM) audio encoding.
+  static void CreateLPCM(const wds::AudioCodec& audio_codec,
+                         const AudioEncoderCallback& encoder_callback);
+
   explicit WiFiDisplayAudioEncoder(const wds::AudioCodec& audio_codec);
   ~WiFiDisplayAudioEncoder() override;
 
   size_t GetAudioCodecMode() const;
 
   const wds::AudioCodec audio_codec_;
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_RENDERER_API_DISPLAY_SOURCE_WIFI_DISPLAY_WIFI_DISPLAY_AUDIO_ENCODER_H_