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We can avoid draining the full marking worklist again by visiting them
before initially draining the worklist. This also preserves the
invariant the the epilogue should not find any new objects.

Bug: chromium:843903
Change-Id: I178d7b564ad9aaf5a086183dad83adfcb5c7d5a1
Reviewed-on: https://chromium-review.googlesource.com/1158412Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579786}