APM: Add build flag to allow building WebRTC without APM

This CL adds a build flag to allow building the non-test parts
of WebRTC without the audio processing module.
The CL also ensures that the WebRTC code correctly handles
the case when no APM is available.

Bug: webrtc:5298
Change-Id: I5c8b5d1f7115e5cce2af4c2b5ff701fa1c54e49e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/171509
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31133}

modules/audio_processing/audio_processing_builder_impl.cc

diff --git a/modules/audio_processing/audio_processing_builder_impl.cc b/modules/audio_processing/audio_processing_builder_impl.cc
new file mode 100644
index 0000000..e89bbec
--- /dev/null
+++ b/modules/audio_processing/audio_processing_builder_impl.cc
@@ -0,0 +1,51 @@
+/*
+ *  Copyright (c) 2020 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 "modules/audio_processing/include/audio_processing.h"
+
+#include <memory>
+
+#include "modules/audio_processing/audio_processing_impl.h"
+#include "rtc_base/ref_counted_object.h"
+
+namespace webrtc {
+
+AudioProcessingBuilder::AudioProcessingBuilder() = default;
+AudioProcessingBuilder::~AudioProcessingBuilder() = default;
+
+AudioProcessing* AudioProcessingBuilder::Create() {
+  webrtc::Config config;
+  return Create(config);
+}
+
+AudioProcessing* AudioProcessingBuilder::Create(const webrtc::Config& config) {
+#ifdef WEBRTC_EXCLUDE_AUDIO_PROCESSING_MODULE
+
+  // Implementation returning a null pointer for using when the APM is excluded
+  // from the build..
+  return nullptr;
+
+#else
+
+  // Standard implementation.
+  AudioProcessingImpl* apm = new rtc::RefCountedObject<AudioProcessingImpl>(
+      config, std::move(capture_post_processing_),
+      std::move(render_pre_processing_), std::move(echo_control_factory_),
+      std::move(echo_detector_), std::move(capture_analyzer_));
+  if (apm->Initialize() != AudioProcessing::kNoError) {
+    delete apm;
+    apm = nullptr;
+  }
+  return apm;
+
+#endif
+}
+
+}  // namespace webrtc