[chrome.displaySource] Add a base class for audio encoders.

extensions/renderer/api/display_source/wifi_display/wifi_display_audio_encoder.cc

+// 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.
+
+#include "extensions/renderer/api/display_source/wifi_display/wifi_display_audio_encoder.h"
+
+#include "base/logging.h"
+
+namespace extensions {
+
+WiFiDisplayAudioEncoder::WiFiDisplayAudioEncoder(
+    const wds::AudioCodec& audio_codec)
+    : audio_codec_(audio_codec) {}
+
+WiFiDisplayAudioEncoder::~WiFiDisplayAudioEncoder() = default;
+
+void WiFiDisplayAudioEncoder::Create(
+    const wds::AudioCodec& audio_codec,
+    const AudioEncoderCallback& encoder_callback) {
+  // Report failure.
+  encoder_callback.Run(nullptr);
+}
+
+size_t WiFiDisplayAudioEncoder::GetAudioCodecMode() const {
+  DCHECK_EQ(1u, audio_codec_.modes.count());
+  for (size_t mode = 0u; mode < audio_codec_.modes.size(); ++mode) {
+    if (audio_codec_.modes.test(mode))
+      return mode;
+  }
+  NOTREACHED();
+  return kInvalidCodecModeValue;
+}
+
+}  // namespace extensions