Revert of [debugger api] remove legacy JSON debug protocol. (patchset #2 id:20001 of https://codereview.chromium.org/2642253005/ )

Reason for revert:
Node.js relies on this

Original issue's description:
> [debugger api] remove legacy JSON debug protocol.
>
> R=jgruber@chromium.org
> BUG=v8:5530
>
> Review-Url: https://codereview.chromium.org/2642253005
> Cr-Commit-Position: refs/heads/master@{#42543}
> Committed: https://chromium.googlesource.com/v8/v8/+/e26a58e43c51a680a0a6363e0066886f4971a41f

TBR=jgruber@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:5530

Review-Url: https://codereview.chromium.org/2644233003
Cr-Commit-Position: refs/heads/master@{#42549}