Change name of files and class in agc/histogram* in order to avoid file-name clash.

webrtc/modules/audio_processing/agc/agc.h

Index: webrtc/modules/audio_processing/agc/agc.h
diff --git a/webrtc/modules/audio_processing/agc/agc.h b/webrtc/modules/audio_processing/agc/agc.h
index 9b6b8556ab95a1a001b7711a834897ed2414dcf9..59faba6866ee3496c5fd3b57fbb0f548f1927ed6 100644
--- a/webrtc/modules/audio_processing/agc/agc.h
+++ b/webrtc/modules/audio_processing/agc/agc.h
@@ -19,7 +19,7 @@
 namespace webrtc {
 
 class AudioFrame;
-class Histogram;
+class LoudnessHistogram;
 
 class Agc {
  public:
@@ -49,8 +49,8 @@ class Agc {
  private:
   double target_level_loudness_;
   int target_level_dbfs_;
-  std::unique_ptr<Histogram> histogram_;
-  std::unique_ptr<Histogram> inactive_histogram_;
+  std::unique_ptr<LoudnessHistogram> histogram_;
+  std::unique_ptr<LoudnessHistogram> inactive_histogram_;
   VoiceActivityDetector vad_;
 };