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This callback is used to report that the DecodeFrame operation is
complete, therefore it's a once callback.

Documentation:
* https://cs.chromium.org/chromium/src/media/cast/receiver/audio_decoder.h?rcl=292ee8149fd602c22de27e034db2fd7b1113cb8d&l=21

The callback is executed via PostTask:
* https://cs.chromium.org/chromium/src/media/cast/receiver/audio_decoder.cc?rcl=292ee8149fd602c22de27e034db2fd7b1113cb8d&l=67
* https://cs.chromium.org/chromium/src/media/cast/receiver/audio_decoder.cc?rcl=292ee8149fd602c22de27e034db2fd7b1113cb8d&l=81

This is part of the base::Callback migration.

Context: https://cs.chromium.org/chromium/src/docs/callback.md?rcl=9fcc3764aea8f97e9f6de4a9ee61d554e67edcda&l=40

Bug: 714018
Change-Id: Id6822683089aa7547171d97a33aa14c0842f97d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2074480
Commit-Queue: Jose Lopes <jabolopes@google.com>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745432}