AGC: Add a histogram for clipping adjustment

This new histogram will log a value every time input clipping is
detected. The value is a boolean, with "true" meaning that the gain
was in fact adjusted in response to the detected clipping, and "false"
meaning that adjustment was not allowed due to kClippedLevelMin.

BUG=webrtc:6622

Review-Url: https://codereview.webrtc.org/2522543006
Cr-Commit-Position: refs/heads/master@{#15196}

webrtc/modules/audio_processing/agc/agc_manager_direct.cc

@@ -21,6 +21,7 @@
 #include "webrtc/modules/audio_processing/gain_control_impl.h"
 #include "webrtc/modules/include/module_common_types.h"
 #include "webrtc/system_wrappers/include/logging.h"
+#include "webrtc/system_wrappers/include/metrics.h"
 
 namespace webrtc {
 
@@ -218,6 +219,8 @@ void AgcManagerDirect::AnalyzePreProcess(int16_t* audio,
     // Always decrease the maximum level, even if the current level is below
     // threshold.
     SetMaxLevel(std::max(kClippedLevelMin, max_level_ - kClippedLevelStep));
+    RTC_HISTOGRAM_BOOLEAN("WebRTC.Audio.AgcClippingAdjustmentAllowed",
+                          level_ - kClippedLevelStep >= kClippedLevelMin);
     if (level_ > kClippedLevelMin) {
       // Don't try to adjust the level if we're already below the limit. As
       // a consequence, if the user has brought the level above the limit, we