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/webrtc.gni b/webrtc.gni
index dcd3400..9750d4d 100644
--- a/webrtc.gni
+++ b/webrtc.gni
@@ -96,6 +96,9 @@
   # should be generated.
   apm_debug_dump = false
 
+  # Selects whether the audio processing module should be excluded.
+  rtc_exclude_audio_processing_module = false
+
   # Set this to true to enable BWE test logging.
   rtc_enable_bwe_test_logging = false