Replace slave -> helper, master -> reference

A slight simplification of the NetEq code is also included.

The subtrees below common_audio, modules/audio_coding and
modules/audio_processing were scanned while making this CL.

Bug: webrtc:11680
Change-Id: I33bb1c75b2e3d1c6793fd1c5741ca59f4b6e8455
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178361
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Henrik Lundin <henrik.lundin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31578}
diff --git a/modules/audio_coding/neteq/time_stretch.cc b/modules/audio_coding/neteq/time_stretch.cc
index 560d9be..ba24e0b 100644
--- a/modules/audio_coding/neteq/time_stretch.cc
+++ b/modules/audio_coding/neteq/time_stretch.cc
@@ -43,7 +43,7 @@
     signal_len = input_len / num_channels_;
     signal_array.reset(new int16_t[signal_len]);
     signal = signal_array.get();
-    size_t j = master_channel_;
+    size_t j = kRefChannel;
     for (size_t i = 0; i < signal_len; ++i) {
       signal_array[i] = input[j];
       j += num_channels_;
@@ -187,7 +187,7 @@
       (static_cast<int64_t>(vec1_energy) + vec2_energy) / 16);
   int32_t right_side;
   if (background_noise_.initialized()) {
-    right_side = background_noise_.Energy(master_channel_);
+    right_side = background_noise_.Energy(kRefChannel);
   } else {
     // If noise parameters have not been estimated, use a fixed threshold.
     right_side = 75000;