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}

pc/BUILD.gn

@@ -587,6 +587,7 @@ if (rtc_include_tests) {
       "../media:rtc_media_engine_defaults",
       "../modules/audio_device:audio_device_api",
       "../modules/audio_processing:audio_processing_statistics",
+      "../modules/audio_processing:audioproc_test_utils",
       "../modules/rtp_rtcp:rtp_rtcp_format",
       "../p2p:fake_ice_transport",
       "../p2p:fake_port_allocator",

pc/peer_connection_integrationtest.cc

@@ -36,6 +36,7 @@
 #include "media/engine/fake_webrtc_video_engine.h"
 #include "media/engine/webrtc_media_engine.h"
 #include "media/engine/webrtc_media_engine_defaults.h"
+#include "modules/audio_processing/test/audio_processing_builder_for_testing.h"
 #include "p2p/base/fake_ice_transport.h"
 #include "p2p/base/mock_async_resolver.h"
 #include "p2p/base/p2p_constants.h"
@@ -648,6 +649,12 @@ class PeerConnectionWrapper : public webrtc::PeerConnectionObserver,
       media_deps.video_decoder_factory.reset();
     }
 
+    if (!media_deps.audio_processing) {
+      // If the standard Creation method for APM returns a null pointer, instead
+      // use the builder for testing to create an APM object.
+      media_deps.audio_processing = AudioProcessingBuilderForTesting().Create();
+    }
+
     pc_factory_dependencies.media_engine =
         cricket::CreateMediaEngine(std::move(media_deps));
     pc_factory_dependencies.call_factory = webrtc::CreateCallFactory();