Use std::abs instead of C-style abs.

webrtc/modules/audio_device/ios/audio_device_ios.mm

Index: webrtc/modules/audio_device/ios/audio_device_ios.mm
diff --git a/webrtc/modules/audio_device/ios/audio_device_ios.mm b/webrtc/modules/audio_device/ios/audio_device_ios.mm
index f1040d8e4ee6e65c8cf4add97c754e7dffe38966..0b01cdc9ad4278f99f5b593d24fc8b818aa35080 100644
--- a/webrtc/modules/audio_device/ios/audio_device_ios.mm
+++ b/webrtc/modules/audio_device/ios/audio_device_ios.mm
@@ -537,7 +537,7 @@ void AudioDeviceIOS::HandleSampleRateChange(float sample_rate) {
          current_sample_rate, (unsigned long)current_frames_per_buffer);;
 
   // Sample rate and buffer size are the same, no work to do.
-  if (abs(current_sample_rate - session_sample_rate) <= DBL_EPSILON &&
+  if (std::abs(current_sample_rate - session_sample_rate) <= DBL_EPSILON &&
       current_frames_per_buffer == session_frames_per_buffer) {
     return;
   }