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/cng/audio_encoder_cng.cc

@@ -14,6 +14,8 @@
 #include <memory>
 #include <utility>
 
+#include "absl/types/optional.h"
+#include "api/units/time_delta.h"
 #include "modules/audio_coding/codecs/cng/webrtc_cng.h"
 #include "rtc_base/checks.h"
 
@@ -55,6 +57,8 @@ class AudioEncoderCng final : public AudioEncoder {
   void OnReceivedUplinkBandwidth(
       int target_audio_bitrate_bps,
       absl::optional<int64_t> bwe_period_ms) override;
+  absl::optional<std::pair<TimeDelta, TimeDelta>> GetFrameLengthRange()
+      const override;
 
  private:
   EncodedInfo EncodePassive(size_t frames_to_encode, rtc::Buffer* encoded);
@@ -225,6 +229,11 @@ void AudioEncoderCng::OnReceivedUplinkBandwidth(
                                              bwe_period_ms);
 }
 
+absl::optional<std::pair<TimeDelta, TimeDelta>>
+AudioEncoderCng::GetFrameLengthRange() const {
+  return speech_encoder_->GetFrameLengthRange();
+}
+
 AudioEncoder::EncodedInfo AudioEncoderCng::EncodePassive(
     size_t frames_to_encode,
     rtc::Buffer* encoded) {