Fall back to default media device from preferences if no default is supplied from the command line.

content/browser/media/media_devices_util.cc

 // Copyright 2017 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 "content/browser/media/media_devices_util.h"
 
 #include <utility>
 #include <vector>
 
 #include "base/bind.h"
@@ skipping 31 matching lines @@
     case MEDIA_DEVICE_TYPE_VIDEO_INPUT:
       media_stream_type = MEDIA_DEVICE_VIDEO_CAPTURE;
       break;
     default:
       return std::string();
   }
 
   return delegate->GetDefaultMediaDeviceID(media_stream_type);
 }
 
-// This function is intended for testing purposes.
+// This function is intended for testing purposes. It returns an empty string
+// if no default device is supplied via the command line.
 std::string GetDefaultMediaDeviceIDFromCommandLine(
     MediaDeviceType device_type) {
   DCHECK(base::CommandLine::ForCurrentProcess()->HasSwitch(
       switches::kUseFakeDeviceForMediaStream));
   const std::string option =
       base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
           switches::kUseFakeDeviceForMediaStream);
   // Optional comma delimited parameters to the command line can specify values
   // for the default device IDs.
   // Examples: "video-input-default-id=mycam, audio-input-default-id=mymic"
@@ skipping 24 matching lines @@
 
 }  // namespace
 
 void GetDefaultMediaDeviceID(
     MediaDeviceType device_type,
     int render_process_id,
     int render_frame_id,
     const base::Callback<void(const std::string&)>& callback) {
   if (base::CommandLine::ForCurrentProcess()->HasSwitch(
           switches::kUseFakeDeviceForMediaStream)) {
-    callback.Run(GetDefaultMediaDeviceIDFromCommandLine(device_type));
-    return;
+    std::string command_line_default_device_id =
+        GetDefaultMediaDeviceIDFromCommandLine(device_type);
+    if (!command_line_default_device_id.empty()) {
+      callback.Run(command_line_default_device_id);
+      return;
+    }
   }
 
   BrowserThread::PostTaskAndReplyWithResult(
       BrowserThread::UI, FROM_HERE,
       base::Bind(&GetDefaultMediaDeviceIDOnUIThread, device_type,
                  render_process_id, render_frame_id),
       callback);
 }
 
 }  // namespace content