Add RuntimeSettings to CustomProcessing.

CustomProcessing is the interface to injectable audio processing
submodules to AudioProcessing. This CL makes it possible to set
runtime settings on the injected render processing component.

Note that the current runtime setting handling happens on the capture
thread. Therefore, we add another SwapQueue to communicate with the
render thread.

Bug: webrtc:9138, webrtc:9262
Change-Id: I665ce2d83a2b35ca8b25cca813d2cef7bd0ba911
Reviewed-on: https://webrtc-review.googlesource.com/76123
Commit-Queue: Alex Loiko <aleloi@webrtc.org>
Reviewed-by: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23236}

modules/audio_processing/include/mock_audio_processing.h

@@ -110,6 +110,8 @@ class MockCustomProcessing : public CustomProcessing {
   virtual ~MockCustomProcessing() {}
   MOCK_METHOD2(Initialize, void(int sample_rate_hz, int num_channels));
   MOCK_METHOD1(Process, void(AudioBuffer* audio));
+  MOCK_METHOD1(SetRuntimeSetting,
+               void(AudioProcessing::RuntimeSetting setting));
   MOCK_CONST_METHOD0(ToString, std::string());
 };