This cl sketches an api for incremental wrapper tracing, but still uses the api
in the stop-the-world fashion. Responsibility to maintain a marking deque is
transfered to the embedder. V8 will still collect wrapper internal fields, but
will send them to the embedder after each incremental gc task. Wrappers must be
sent at latest by the time next oilpan gc runs - so blink can keep all
discovered wrappables alive.

The old api will be cleared after this cl and corresponding blink cl land.

Comments are very welcomed :)

LOG=no
BUG=468240

Review-Url: https://codereview.chromium.org/2032213003
Cr-Commit-Position: refs/heads/master@{#36761}