AEC3: Add signal dependent mixing before alignment

This CL adds code for doing signal-dependent downmixing
before the delay estimation in the multichannel case.

As part of the CL, the unittests of the render delay
controller are corrected. However, as that caused some of
them to fail, the CL (for now) as well disables the failing
test.

Bug: webrtc:11153,chromium:1029740, webrtc:11161
Change-Id: I0b765c28fa5e547aabd6dfbd24b626ff9a16346f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/161045
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29980}

modules/audio_processing/aec3/render_delay_controller.h

@@ -25,7 +25,8 @@ namespace webrtc {
 class RenderDelayController {
  public:
   static RenderDelayController* Create(const EchoCanceller3Config& config,
-                                       int sample_rate_hz);
+                                       int sample_rate_hz,
+                                       size_t num_capture_channels);
   virtual ~RenderDelayController() = default;
 
   // Resets the delay controller. If the delay confidence is reset, the reset