Revert of [debugger] deprecate v8::Debug:GetDebugContext. (patchset #2 id:20001 of https://codereview.chromium.org/2589203002/ )

Reason for revert:
crbug/676749

Original issue's description:
> [debugger] deprecate v8::Debug:GetDebugContext.
>
> R=jgruber@chromium.org
> BUG=v8:5530
>
> Review-Url: https://codereview.chromium.org/2589203002
> Cr-Commit-Position: refs/heads/master@{#41911}
> Committed: https://chromium.googlesource.com/v8/v8/+/381082168d37d4f2794360ae31dc078f2a350113

TBR=jgruber@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=v8:5530, chromium:676749

Review-Url: https://codereview.chromium.org/2595413003
Cr-Commit-Position: refs/heads/master@{#41935}