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

Index: webrtc/modules/audio_device/linux/audio_device_pulse_linux.cc
diff --git a/webrtc/modules/audio_device/linux/audio_device_pulse_linux.cc b/webrtc/modules/audio_device/linux/audio_device_pulse_linux.cc
index 8b0ae6f53906f6f27cb8bd4a09a554e20a0cd851..3bbc1859d8dfd477cb333e052b593184fac737c7 100644
--- a/webrtc/modules/audio_device/linux/audio_device_pulse_linux.cc
+++ b/webrtc/modules/audio_device/linux/audio_device_pulse_linux.cc
@@ -891,7 +891,6 @@ int32_t AudioDeviceLinuxPulse::MicrophoneVolumeStepSize(
 
 int16_t AudioDeviceLinuxPulse::PlayoutDevices()
 {
-    DCHECK(thread_checker_.CalledOnValidThread());
     PaLock();
 
     pa_operation* paOperation = NULL;
@@ -1029,7 +1028,6 @@ int32_t AudioDeviceLinuxPulse::RecordingDeviceName(
 
 int16_t AudioDeviceLinuxPulse::RecordingDevices()
 {
-    DCHECK(thread_checker_.CalledOnValidThread());
     PaLock();
 
     pa_operation* paOperation = NULL;