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This callback is used to signal completion of the read operation,
therefore, it's a once callback:
* https://cs.chromium.org/chromium/src/media/base/data_source.h?rcl=6e8610f8f31aa3bb8266660380a66c6c25b2e64a&l=27

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: Ie43febda1247be065f9f22cc4aec0d8412873491
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2056729Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Jose Lopes <jabolopes@google.com>
Cr-Commit-Position: refs/heads/master@{#745018}