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The MojoSharedBufferDoneCB callback is a once callback because it's executed
only once during the class's constructor:
* https://cs.chromium.org/chromium/src/media/mojo/common/mojo_shared_buffer_video_frame.cc?rcl=8184a68c47988fb128372c9c688f77455dfcb5db&l=250

This is also documented in its definition:
* https://cs.chromium.org/chromium/src/media/mojo/common/mojo_shared_buffer_video_frame.cc?rcl=8184a68c47988fb128372c9c688f77455dfcb5db&l=250

The callback is passed in the TransformToVideoFrame method, in:
* https://cs.chromium.org/chromium/src/media/mojo/common/mojo_shared_buffer_video_frame.cc?rcl=8184a68c47988fb128372c9c688f77455dfcb5db&l=250

and this method can only be called once. The evidence of the following statements:
* DCHECK(FrameBuffer()): https://cs.chromium.org/chromium/src/media/mojo/services/mojo_cdm_allocator.cc?rcl=8184a68c47988fb128372c9c688f77455dfcb5db&l=116
* SetFrameBuffer(nullptr): https://cs.chromium.org/chromium/src/media/mojo/services/mojo_cdm_allocator.cc?rcl=8184a68c47988fb128372c9c688f77455dfcb5db&l=116

Together these guarantee that the method can only be called once.

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: Ib4dff4d9c491682b8abd134723ef88bcf5252bc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2069540
Commit-Queue: Jose Lopes <jabolopes@google.com>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744610}