With cl https://codereview.chromium.org/2043033002 blink will add new wrappers
to its marking deque (and maintain this deque) so black allocated wrappers are
not a problem anymore. To limit the scope of when blink has to detect wrappers,
we need to tell blink when we start and when we end full gcs. For that we can
nicely use TracePrologue and TraceEpilogue.

LOG=no
BUG=468240

Review-Url: https://codereview.chromium.org/2051103002
Cr-Commit-Position: refs/heads/master@{#36937}