Oilpan: Avoid tracing HeapVector twice
Oilpan: Avoid tracing HeapVector twice

If a pointer to the HeapVector is found by conservative scanning,
the HeapVector can trace its Members twice (once in the conservative scanning,
and then in visitor->trace()). This is just redundant.

This CL adds a check to avoid tracing HeapVector twice.
The same optimization is already implemented in HeapHashTable.

BUG=475801

Review URL: https://codereview.chromium.org/1155443002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@195681 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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