Revert of deprecate non-phantom weak callbacks (patchset #1 id:1 of https://codereview.chromium.org/1103173002/)

Reason for revert:
[Sheriff] This is still used and now blocks the roll here:
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_compile_dbg_ng/builds/47889

Original issue's description:
> Reland: deprecate non-phantom weak callbacks
>
> BUG=
>
> Committed: https://crrev.com/3be656fb55ad49834ec8ec0a57ba858bb1b21f37
> Cr-Commit-Position: refs/heads/master@{#28094}

TBR=jochen@chromium.org,dcarney@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1117623002

Cr-Commit-Position: refs/heads/master@{#28121}