Sort camera device list to use front camera first

media/capture/video/video_capture_device_descriptor.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 MEDIA_CAPTURE_VIDEO_VIDEO_CAPTURE_DEVICE_DESCRIPTOR_H_
 #define MEDIA_CAPTURE_VIDEO_VIDEO_CAPTURE_DEVICE_DESCRIPTOR_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 49 matching lines @@
   VideoCaptureDeviceDescriptor(const VideoCaptureDeviceDescriptor& other);
   ~VideoCaptureDeviceDescriptor();
 
   // These operators are needed due to storing the name in an STL container.
   // In the shared build, all methods from the STL container will be exported
   // so even though they're not used, they're still depended upon.
   bool operator==(const VideoCaptureDeviceDescriptor& other) const {
     return (other.device_id == device_id) && (other.capture_api == capture_api);
   }
   bool operator<(const VideoCaptureDeviceDescriptor& other) const {
+    // Front camera first

[wuchengli, 2017/01/24 04:25:09]

We should prefer on-device cameras over external cameras. Front > back > none. 

[henryhsu, 2017/01/24 04:55:45]

Done.

+    if (facing < other.facing)
+      return true;
     if (device_id < other.device_id)
       return true;
     return capture_api < other.capture_api;
   }
 
   const char* GetCaptureApiTypeString() const;
   // Friendly name of a device, plus the model identifier in parentheses.
   std::string GetNameAndModel() const;
 
   std::string display_name;  // Name that is intended for display in the UI
   std::string device_id;
   // A unique hardware identifier of the capture device.
   // It is of the form "[vid]:[pid]" when a USB device is detected, and empty
   // otherwise.
   std::string model_id;
 
   VideoFacingMode facing;
 
   VideoCaptureApi capture_api;
   VideoCaptureTransportType transport_type;
 };
 
 using VideoCaptureDeviceDescriptors = std::vector<VideoCaptureDeviceDescriptor>;
 
 }  // namespace media
 
 #endif  // MEDIA_CAPTURE_VIDEO_VIDEO_CAPTURE_DEVICE_DESCRIPTOR_H_