RTCVideoEncoder: Rename output_buffers_free_count_

The description about |output_buffers_free_count_| of
RTCVIdeoEncoder::Impl is "The number of output buffers ready to be
filled with output from the encoder". It is unclear what state the
buffer is in. It represents the number of output buffers that are sent
to a hardware video encoder by UseOutputBitstreamBuffer() and the
hardware encoder holds them. This CL renames the variable to
|output_buffers_in_encoder_cnt_| for clarification.

Bug: None
Test: Build

Change-Id: I10dc8f52ac826ae8368623bfe9633d597ead3df5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4263928
Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1107484}