Since the generic GetCallingContext is deprecated, but there's still the
use case for the debugger to get the currently debugged context while in
the debug context, add a convenience API for it.

Note that EventDetails already exposes this context, but the embedder
might not necessarily have the EventDetails around.

R=verwaest@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2040853003
Cr-Commit-Position: refs/heads/master@{#36751}