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This callback is meant to be called every time a new key is registered,
thus it must be a repeating callback, as documented in:
https://cs.chromium.org/chromium/src/media/base/decryptor.h?rcl=502d7a8bc634c4248b7cc2f43e2305eee7213d94&l=51

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: Ib852f03ab2f862d02d0e09be95626090cee60d75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2035960
Commit-Queue: Jose Lopes <jabolopes@google.com>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743063}