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- Avoid tracing elements directly from WTF::Vector and WTF::Deque
  when the backing store is allocated out-of-line.
- Avoid MarkNoTrace optimization breaking the tracing/marking
  abstraction.

Bug: chromium:802273
Change-Id: Icfb5deed3479d4a7dec85c82930b464bdfe16b0e
Reviewed-on: https://chromium-review.googlesource.com/886484
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532806}