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}
diff --git a/modules/audio_processing/test/debug_dump_test.cc b/modules/audio_processing/test/debug_dump_test.cc
index 71478a9..2381d1e 100644
--- a/modules/audio_processing/test/debug_dump_test.cc
+++ b/modules/audio_processing/test/debug_dump_test.cc
@@ -17,6 +17,7 @@
 #include "api/audio/echo_canceller3_factory.h"
 #include "modules/audio_coding/neteq/tools/resample_input_audio_file.h"
 #include "modules/audio_processing/aec_dump/aec_dump_factory.h"
+#include "modules/audio_processing/test/audio_processing_builder_for_testing.h"
 #include "modules/audio_processing/test/debug_dump_replayer.h"
 #include "modules/audio_processing/test/test_utils.h"
 #include "rtc_base/task_queue_for_test.h"
@@ -141,7 +142,7 @@
       enable_pre_amplifier_(enable_pre_amplifier),
       worker_queue_("debug_dump_generator_worker_queue"),
       dump_file_name_(dump_file_name) {
-  AudioProcessingBuilder apm_builder;
+  AudioProcessingBuilderForTesting apm_builder;
   apm_.reset(apm_builder.Create(config));
 }