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This is a once callback because it is run whe the EncodeVideoFrame
method completes, as documented in:
* https://cs.chromium.org/chromium/src/media/cast/sender/video_encoder.h?l=27&gsn=FrameEncodedCallback
* https://cs.chromium.org/chromium/src/media/cast/sender/video_encoder.h?rcl=d00d2b00c0d2b47044ea1816d51ad559d2bdcfd3&l=48

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: I3e62ce9a72e632b0aa87dd2d3f839e862b63af6c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2074503
Auto-Submit: Jose Lopes <jabolopes@google.com>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747031}