Reland Process 8 kHz audio as 16 kHz internally of the audio processing module

This CL relands the code from the CL "Process 8 kHz audio as 16 kHz internally
of the audio processing module" which by mistake was reverted via a rebase in
another CL.

The CL changes the behavior of APM for 8 kHz so that it is internally
processed as 16 kHz.

Bug: webrtc:10863
Change-Id: I32a57b2d279c2134125667c19b09cfda381a33c3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150221
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28944}

modules/audio_processing/include/audio_processing.h

@@ -698,6 +698,7 @@ class AudioProcessing : public rtc::RefCountInterface {
     kBadStreamParameterWarning = -13
   };
 
+  // Native rates supported by the AudioFrame interfaces.
   enum NativeRate {
     kSampleRate8kHz = 8000,
     kSampleRate16kHz = 16000,