Allow AEC3 to use any externally reported audio buffer delay in AEC3

This CL adds support for using any externally reported audio buffer
delay to set the initial alignment in AEC3 which is used before the
AEC has been able to detect the delay.

Bug: chromium:834182,webrtc:9163
Change-Id: Ic71355f69b7c4d5815b78e49987043441e7908fb
Reviewed-on: https://webrtc-review.googlesource.com/70580
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Commit-Queue: Gustaf Ullberg <gustaf@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22917}

modules/audio_processing/aec3/block_processor.h

@@ -42,6 +42,9 @@ class BlockProcessor {
   // Get current metrics.
   virtual void GetMetrics(EchoControl::Metrics* metrics) const = 0;
 
+  // Provides an optional external estimate of the audio buffer delay.
+  virtual void SetAudioBufferDelay(size_t delay_ms) = 0;
+
   // Processes a block of capture data.
   virtual void ProcessCapture(
       bool echo_path_gain_change,