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The contents moved to callback_helpers.h.

Also renames a few new bind_helpers.h includes that have been added
in the meantime.

R=dcheng@chromium.org
TBR=

Change-Id: Ia511a4939d60bd2ef5b474185e3716017543f8af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523545
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826387}