Add FixedGainController and move GainController2 in APM.

The FixedGainController (FGC) applies a fixed gain. It will also
control the limiter. The limiter will be landed over the next several
CLs.

The GainController2 is a 'private submodule' of APM. It will control
the new automatic gain controller (AGC). It controls the AGC through
Initialize() and ApplyConfig().

This CL contains

* build changes to make modules/audio_processing/agc2 an independent
  target

* a new MutableFloatAudioFrame which is the audio interface between
  AGC2 and APM

* move of the fixed gain application from GainController2 to
  FixedGainController.

If you are a googler, there is more information in this doc:
https://docs.google.com/document/d/1RV2Doet3MZtUPAHVva61Vjo20iyd1bmmm3aR8znWpzo/edit#

Bug: webrtc:7949
Change-Id: Ief95cbbce83c3aafe54638fd2ab881c9fb8bdc3a
Reviewed-on: https://webrtc-review.googlesource.com/50440
Commit-Queue: Alex Loiko <aleloi@webrtc.org>
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22046}

modules/audio_processing/gain_controller2_unittest.cc

@@ -0,0 +1,98 @@
+/*
+ *  Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include <algorithm>
+
+#include "api/array_view.h"
+#include "modules/audio_processing/audio_buffer.h"
+#include "modules/audio_processing/gain_controller2.h"
+#include "rtc_base/checks.h"
+#include "test/gtest.h"
+
+namespace webrtc {
+namespace test {
+
+namespace {
+
+constexpr size_t kFrameSizeMs = 10u;
+constexpr size_t kStereo = 2u;
+
+void SetAudioBufferSamples(float value, AudioBuffer* ab) {
+  // Sets all the samples in |ab| to |value|.
+  for (size_t k = 0; k < ab->num_channels(); ++k) {
+    std::fill(ab->channels_f()[k], ab->channels_f()[k] + ab->num_frames(),
+              value);
+  }
+}
+
+}  // namespace
+
+TEST(GainController2, CreateApplyConfig) {
+  // Instances GainController2 and applies different configurations.
+  std::unique_ptr<GainController2> gain_controller2(new GainController2());
+
+  // Check that the default config is valid.
+  AudioProcessing::Config::GainController2 config;
+  EXPECT_TRUE(GainController2::Validate(config));
+  gain_controller2->ApplyConfig(config);
+
+  // Check that attenuation is not allowed.
+  config.fixed_gain_db = -5.f;
+  EXPECT_FALSE(GainController2::Validate(config));
+
+  // Check that valid configurations are applied.
+  for (const float& fixed_gain_db : {0.f, 5.f, 10.f, 50.f}) {
+    config.fixed_gain_db = fixed_gain_db;
+    EXPECT_TRUE(GainController2::Validate(config));
+    gain_controller2->ApplyConfig(config);
+  }
+}
+
+TEST(GainController2, ToString) {
+  // Tests GainController2::ToString().
+  AudioProcessing::Config::GainController2 config;
+  config.fixed_gain_db = 5.f;
+
+  config.enabled = false;
+  config.enable_limiter = true;
+  EXPECT_EQ("{enabled: false, fixed_gain_dB: 5, enable_limiter: true}",
+            GainController2::ToString(config));
+
+  config.enabled = true;
+  EXPECT_EQ("{enabled: true, fixed_gain_dB: 5, enable_limiter: true}",
+            GainController2::ToString(config));
+}
+
+TEST(GainController2, Usage) {
+  // Tests GainController2::Process() on an AudioBuffer instance.
+  std::unique_ptr<GainController2> gain_controller2(new GainController2());
+  gain_controller2->Initialize(AudioProcessing::kSampleRate48kHz);
+  const size_t num_frames = rtc::CheckedDivExact<size_t>(
+      kFrameSizeMs * AudioProcessing::kSampleRate48kHz, 1000);
+  AudioBuffer ab(num_frames, kStereo, num_frames, kStereo, num_frames);
+  constexpr float sample_value = 1000.f;
+  SetAudioBufferSamples(sample_value, &ab);
+  AudioProcessing::Config::GainController2 config;
+
+  // Check that samples are not modified when the fixed gain is 0 dB.
+  config.fixed_gain_db = 0.f;
+  gain_controller2->ApplyConfig(config);
+  gain_controller2->Process(&ab);
+  EXPECT_EQ(ab.channels_f()[0][0], sample_value);
+
+  // Check that samples are amplified when the fixed gain is greater than 0 dB.
+  config.fixed_gain_db = 5.f;
+  gain_controller2->ApplyConfig(config);
+  gain_controller2->Process(&ab);
+  EXPECT_LT(sample_value, ab.channels_f()[0][0]);
+}
+
+}  // namespace test
+}  // namespace webrtc