Implement AudioEncoder::GetFrameLengthRange() for all audio encoders.

The WebRTC-SendSideBwe-WithOverhead field trial requires audio
encoders to properly implement the
AudioEncoder::GetFrameLengthRange() function. Thic CL implements
the function for all audio encoders in WebRTC in preparation for
making that function pure virtual in the interface.


Bug: webrtc:11427
Change-Id: Ieab6b6c72c62af6ac9525a20fcb39bd477079551
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/171503
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Minyue Li <minyue@webrtc.org>
Commit-Queue: Minyue Li <minyue@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30890}

modules/audio_coding/codecs/g711/audio_encoder_pcm.h

@@ -11,9 +11,12 @@
 #ifndef MODULES_AUDIO_CODING_CODECS_G711_AUDIO_ENCODER_PCM_H_
 #define MODULES_AUDIO_CODING_CODECS_G711_AUDIO_ENCODER_PCM_H_
 
+#include <utility>
 #include <vector>
 
+#include "absl/types/optional.h"
 #include "api/audio_codecs/audio_encoder.h"
+#include "api/units/time_delta.h"
 #include "rtc_base/constructor_magic.h"
 
 namespace webrtc {
@@ -41,6 +44,8 @@ class AudioEncoderPcm : public AudioEncoder {
   size_t Max10MsFramesInAPacket() const override;
   int GetTargetBitrate() const override;
   void Reset() override;
+  absl::optional<std::pair<TimeDelta, TimeDelta>> GetFrameLengthRange()
+      const override;
 
  protected:
   AudioEncoderPcm(const Config& config, int sample_rate_hz);