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/ilbc/audio_encoder_ilbc.h

@@ -11,8 +11,12 @@
 #ifndef MODULES_AUDIO_CODING_CODECS_ILBC_AUDIO_ENCODER_ILBC_H_
 #define MODULES_AUDIO_CODING_CODECS_ILBC_AUDIO_ENCODER_ILBC_H_
 
+#include <utility>
+
+#include "absl/types/optional.h"
 #include "api/audio_codecs/audio_encoder.h"
 #include "api/audio_codecs/ilbc/audio_encoder_ilbc_config.h"
+#include "api/units/time_delta.h"
 #include "modules/audio_coding/codecs/ilbc/ilbc.h"
 #include "rtc_base/constructor_magic.h"
 
@@ -32,6 +36,8 @@ class AudioEncoderIlbcImpl final : public AudioEncoder {
                          rtc::ArrayView<const int16_t> audio,
                          rtc::Buffer* encoded) override;
   void Reset() override;
+  absl::optional<std::pair<TimeDelta, TimeDelta>> GetFrameLengthRange()
+      const override;
 
  private:
   size_t RequiredOutputSizeBytes() const;