Dropping too many consecutive input buffers reduces the update
frequency of MCVR.shouldDropDecoderInputBuffers and can lead to
dropping too many consecutive input buffers.
Discarding input buffers of type OBU_FRAME_HEADER with
show_existing_frame = 1 saves a smaller amount of resources
than discarding input buffers of type OBU_FRAME.
PiperOrigin-RevId: 730362707
(cherry picked from commit 67e99f46481dbc5fff9ffd04ff13119dac9199bb)