Initialize default audio device ID with explicit device ID.

content/renderer/media/user_media_client_impl_unittest.cc

Index: content/renderer/media/user_media_client_impl_unittest.cc
diff --git a/content/renderer/media/user_media_client_impl_unittest.cc b/content/renderer/media/user_media_client_impl_unittest.cc
index 9735d97389a939bdd89a5aa20ef560c0465f2077..f01ef2dc546bfa9c9bd4d1e102123093cd4f04a3 100644
--- a/content/renderer/media/user_media_client_impl_unittest.cc
+++ b/content/renderer/media/user_media_client_impl_unittest.cc
@@ -22,6 +22,7 @@
 #include "content/renderer/media/mock_media_stream_dispatcher.h"
 #include "content/renderer/media/mock_media_stream_video_source.h"
 #include "content/renderer/media/webrtc/mock_peer_connection_dependency_factory.h"
+#include "media/audio/audio_device_description.h"
 #include "mojo/public/cpp/bindings/binding.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "third_party/WebKit/public/platform/WebMediaDeviceInfo.h"
@@ -900,11 +901,12 @@ TEST_F(UserMediaClientImplTest, CreateWithAllOptionalInvalidDeviceId) {
       CreateDeviceConstraints(nullptr, kInvalidDeviceId, kInvalidDeviceId);
   blink::WebMediaConstraints video_constraints =
       CreateDeviceConstraints(nullptr, kInvalidDeviceId, kInvalidDeviceId);
-  // MockMediaStreamDispatcher uses empty string as default audio device ID.
   // MockMediaDevicesDispatcher uses the first device in the enumeration as
   // default video device ID.
-  TestValidRequestWithConstraints(audio_constraints, video_constraints,
-                                  std::string(), kFakeVideoInputDeviceId1);
+  TestValidRequestWithConstraints(
+      audio_constraints, video_constraints,
+      std::string(media::AudioDeviceDescription::kDefaultDeviceId),
+      kFakeVideoInputDeviceId1);
 }
 
 TEST_F(UserMediaClientImplTest, CreateWithFacingModeUser) {