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When a refcounted object is constructed, it's refcount is initally zero.
This is a problem if the constructor can post a task to another thread
bacause the scoped_refptr destructor on the other thread might run before
the posting thread has had a chance to increment the refcount.  This
results in a UAF on the posting thread.

The work around is to ensure the refcount is not zero before posting.

Bug: 966964, 906125
Change-Id: Idf3163f979a49abe39088f5af72e94dee4492833
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628758
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664715}