Enable cpplint and fix cpplint errors in webrtc/*audio

webrtc/common_audio/audio_converter.cc

Index: webrtc/common_audio/audio_converter.cc
diff --git a/webrtc/common_audio/audio_converter.cc b/webrtc/common_audio/audio_converter.cc
index 231ba88077dd05dd953d493794231eac0ae9556f..e8595333f6ded05fc4bb281dac38893c92113ca0 100644
--- a/webrtc/common_audio/audio_converter.cc
+++ b/webrtc/common_audio/audio_converter.cc
@@ -107,7 +107,8 @@ class ResampleConverter : public AudioConverter {
 // converters must be provided.
 class CompositionConverter : public AudioConverter {
  public:
-  CompositionConverter(std::vector<std::unique_ptr<AudioConverter>> converters)
+  explicit CompositionConverter(
+    std::vector<std::unique_ptr<AudioConverter>> converters)
       : converters_(std::move(converters)) {
     RTC_CHECK_GE(converters_.size(), 2);
     // We need an intermediate buffer after every converter.