Fix the problom that on Linux no external audio device can be selected since #9243.

webrtc/modules/audio_device/linux/audio_device_pulse_linux.cc

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 873 matching lines @@
         return -1;
     }
 
     stepSize = delta;
 
     return 0;
 }
 
 int16_t AudioDeviceLinuxPulse::PlayoutDevices()
 {
-    DCHECK(thread_checker_.CalledOnValidThread());
     PaLock();
 
     pa_operation* paOperation = NULL;
     _numPlayDevices = 1; // init to 1 to account for "default"
 
     // get the whole list of devices and update _numPlayDevices
     paOperation = LATE(pa_context_get_sink_info_list)(_paContext,
                                                       PaSinkInfoCallback,
                                                       this);
 
@@ skipping 117 matching lines @@
 
     // Clear device name and index
     _recDisplayDeviceName = NULL;
     _deviceIndex = -1;
 
     return 0;
 }
 
 int16_t AudioDeviceLinuxPulse::RecordingDevices()
 {
-    DCHECK(thread_checker_.CalledOnValidThread());
     PaLock();
 
     pa_operation* paOperation = NULL;
     _numRecDevices = 1; // Init to 1 to account for "default"
 
     // Get the whole list of devices and update _numRecDevices
     paOperation = LATE(pa_context_get_source_info_list)(_paContext,
                                                         PaSourceInfoCallback,
                                                         this);
 
@@ skipping 1972 matching lines @@
 
   // A bit change in keymap means a key is pressed
   for (i = 0; i < sizeof(szKey); i++)
     state |= (szKey[i] ^ _oldKeyState[i]) & szKey[i];
 
   // Save old state
   memcpy((char*)_oldKeyState, (char*)szKey, sizeof(_oldKeyState));
   return (state != 0);
 }
 }