Add new experiment code for TunedClippingLevelMin

This CL wires up the experiment settings for tuning WebRTC's gain
control. Four different new values are provided as feature flags (to
be controlled by a Finch experiment or directly from command line
flags).

BUG=chromium:672476,webrtc:6622

Review-Url: https://codereview.chromium.org/2560173002
Cr-Commit-Position: refs/heads/master@{#437847}

content/renderer/media/media_stream_audio_processor.cc

@@ -9,8 +9,10 @@
 #include <utility>
 
 #include "base/command_line.h"
+#include "base/feature_list.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram_macros.h"
+#include "base/optional.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/threading/thread_task_runner_handle.h"
@@ -106,13 +108,43 @@ void RecordProcessingState(AudioTrackProcessingStates state) {
 
 // Checks if the default minimum starting volume value for the AGC is overridden
 // on the command line.
-bool GetStartupMinVolumeForAgc(int* startup_min_volume) {
-  DCHECK(startup_min_volume);
+base::Optional<int> GetStartupMinVolumeForAgc() {
   std::string min_volume_str(
       base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
           switches::kAgcStartupMinVolume));
-  return !min_volume_str.empty() &&
-         base::StringToInt(min_volume_str, startup_min_volume);
+  int startup_min_volume;
+  if (min_volume_str.empty() ||
+      !base::StringToInt(min_volume_str, &startup_min_volume)) {
+    return base::Optional<int>();
+  }
+  return base::Optional<int>(startup_min_volume);
+}
+
+// Features for http://crbug.com/672476. These values will be given to WebRTC's
+// gain control (AGC) as lower bounds for the gain reduction during clipping.
+const base::Feature kTunedClippingLevelMin30{
+    "TunedClippingLevelMin30", base::FEATURE_DISABLED_BY_DEFAULT};
+const base::Feature kTunedClippingLevelMin70{
+    "TunedClippingLevelMin70", base::FEATURE_DISABLED_BY_DEFAULT};
+const base::Feature kTunedClippingLevelMin110{
+    "TunedClippingLevelMin110", base::FEATURE_DISABLED_BY_DEFAULT};
+const base::Feature kTunedClippingLevelMin150{
+    "TunedClippingLevelMin150", base::FEATURE_DISABLED_BY_DEFAULT};
+const base::Feature kTunedClippingLevelMin170{
+    "TunedClippingLevelMin170", base::FEATURE_DISABLED_BY_DEFAULT};
+
+base::Optional<int> GetClippingLevelMin() {
+  if (base::FeatureList::IsEnabled(kTunedClippingLevelMin30))
+    return base::Optional<int>(30);
+  if (base::FeatureList::IsEnabled(kTunedClippingLevelMin70))
+    return base::Optional<int>(70);
+  if (base::FeatureList::IsEnabled(kTunedClippingLevelMin110))
+    return base::Optional<int>(110);
+  if (base::FeatureList::IsEnabled(kTunedClippingLevelMin150))
+    return base::Optional<int>(150);
+  if (base::FeatureList::IsEnabled(kTunedClippingLevelMin170))
+    return base::Optional<int>(170);
+  return base::Optional<int>();
 }
 
 // Checks if the AEC's refined adaptive filter tuning was enabled on the command
@@ -620,10 +652,12 @@ void MediaStreamAudioProcessor::InitializeAudioProcessingModule(
 
   // If the experimental AGC is enabled, check for overridden config params.
   if (audio_constraints.GetGoogExperimentalAutoGainControl()) {
-    int startup_min_volume = 0;
-    if (GetStartupMinVolumeForAgc(&startup_min_volume)) {
-      config.Set<webrtc::ExperimentalAgc>(
-          new webrtc::ExperimentalAgc(true, startup_min_volume));
+    auto startup_min_volume = GetStartupMinVolumeForAgc();
+    auto clipping_level_min = GetClippingLevelMin();
+    if (startup_min_volume || clipping_level_min) {
+      config.Set<webrtc::ExperimentalAgc>(new webrtc::ExperimentalAgc(
+          true, startup_min_volume.value_or(0),
+          clipping_level_min.value_or(webrtc::kClippedLevelMin)));
     }
   }