Initialize default audio device ID with explicit device ID.

content/renderer/media/user_media_client_impl.cc

Index: content/renderer/media/user_media_client_impl.cc
diff --git a/content/renderer/media/user_media_client_impl.cc b/content/renderer/media/user_media_client_impl.cc
index 100cb5368675761e28b8168000a964df5c0f47ee..cb5d7bea7d4721eac62b9bff3df0c7986ece2f9f 100644
--- a/content/renderer/media/user_media_client_impl.cc
+++ b/content/renderer/media/user_media_client_impl.cc
@@ -31,6 +31,7 @@
 #include "content/renderer/media/webrtc_logging.h"
 #include "content/renderer/media/webrtc_uma_histograms.h"
 #include "content/renderer/render_thread_impl.h"
+#include "media/audio/audio_device_description.h"
 #include "media/capture/video_capture_types.h"
 #include "third_party/WebKit/public/platform/WebMediaConstraints.h"
 #include "third_party/WebKit/public/platform/WebMediaDeviceInfo.h"
@@ -85,7 +86,7 @@ bool PickDeviceId(const blink::WebMediaConstraints& constraints,
                   const MediaDeviceInfoArray& device_infos,
                   std::string* device_id) {
   DCHECK(!constraints.IsNull());
-  DCHECK(device_id->empty());
+  DCHECK(media::AudioDeviceDescription::IsDefaultDevice(*device_id));
 
   if (constraints.Basic().device_id.Exact().size() > 1) {
     LOG(ERROR) << "Only one required device ID is supported";
@@ -398,6 +399,12 @@ void UserMediaClientImpl::MaybeProcessNextRequestInfo() {
         current_request_info_->request().AudioConstraints(),
         &current_request_info_->stream_controls()->audio,
         &request_audio_input_devices);
+    // Explicitly initialize the requested device ID to the default.
+    if (IsDeviceSource(
+            current_request_info_->stream_controls()->audio.stream_source)) {
+      current_request_info_->stream_controls()->audio.device_id =
+          std::string(media::AudioDeviceDescription::kDefaultDeviceId);
+    }
     CopyHotwordAndLocalEchoToStreamControls(
         current_request_info_->request().AudioConstraints(),
         current_request_info_->stream_controls());