Fix compile error in audio_manager_cras when dcheck_always_on=true

media/audio/cras/audio_manager_cras.cc

Index: media/audio/cras/audio_manager_cras.cc
diff --git a/media/audio/cras/audio_manager_cras.cc b/media/audio/cras/audio_manager_cras.cc
index 9ba37466cdfacaac69c22c26f048e88cf21cb731..2e6d346e73bb1d47d5c343911e4bcb6417d079ac 100644
--- a/media/audio/cras/audio_manager_cras.cc
+++ b/media/audio/cras/audio_manager_cras.cc
@@ -92,7 +92,7 @@ std::string MicPositions() {
 // virtual device name for them.
 void ProcessVirtualDeviceName(AudioDeviceNames* device_names,
                               const chromeos::AudioDeviceList& device_list) {
-  DCHECK_EQ(2, device_list.size());
+  DCHECK_EQ(2U, device_list.size());
   if (device_list[0].type == chromeos::AUDIO_TYPE_LINEOUT ||
       device_list[1].type == chromeos::AUDIO_TYPE_LINEOUT) {
     device_names->emplace_back(kHeadphoneLineOutVirtualDevice,