We used to reserve the 0-th embedder data field for the debug
context id. This is no longer necessary since the inspector
has migrated to be part of V8. This makes the API a bit simpler.

R=clemensh@chromium.org, jochen@chromium.org, kozyatinskiy@chromium.org
BUG=v8:5530

Review-Url: https://codereview.chromium.org/2806303002
Cr-Commit-Position: refs/heads/master@{#44607}