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/include/audio_processing.h

@@ -685,19 +685,34 @@ class RTC_EXPORT AudioProcessingBuilder {
   ~AudioProcessingBuilder();
   // The AudioProcessingBuilder takes ownership of the echo_control_factory.
   AudioProcessingBuilder& SetEchoControlFactory(
-      std::unique_ptr<EchoControlFactory> echo_control_factory);
+      std::unique_ptr<EchoControlFactory> echo_control_factory) {
+    echo_control_factory_ = std::move(echo_control_factory);
+    return *this;
+  }
   // The AudioProcessingBuilder takes ownership of the capture_post_processing.
   AudioProcessingBuilder& SetCapturePostProcessing(
-      std::unique_ptr<CustomProcessing> capture_post_processing);
+      std::unique_ptr<CustomProcessing> capture_post_processing) {
+    capture_post_processing_ = std::move(capture_post_processing);
+    return *this;
+  }
   // The AudioProcessingBuilder takes ownership of the render_pre_processing.
   AudioProcessingBuilder& SetRenderPreProcessing(
-      std::unique_ptr<CustomProcessing> render_pre_processing);
+      std::unique_ptr<CustomProcessing> render_pre_processing) {
+    render_pre_processing_ = std::move(render_pre_processing);
+    return *this;
+  }
   // The AudioProcessingBuilder takes ownership of the echo_detector.
   AudioProcessingBuilder& SetEchoDetector(
-      rtc::scoped_refptr<EchoDetector> echo_detector);
+      rtc::scoped_refptr<EchoDetector> echo_detector) {
+    echo_detector_ = std::move(echo_detector);
+    return *this;
+  }
   // The AudioProcessingBuilder takes ownership of the capture_analyzer.
   AudioProcessingBuilder& SetCaptureAnalyzer(
-      std::unique_ptr<CustomAudioAnalyzer> capture_analyzer);
+      std::unique_ptr<CustomAudioAnalyzer> capture_analyzer) {
+    capture_analyzer_ = std::move(capture_analyzer);
+    return *this;
+  }
   // This creates an APM instance using the previously set components. Calling
   // the Create function resets the AudioProcessingBuilder to its initial state.
   AudioProcessing* Create();