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The CancelIo() operation does not synchronously cancel IO operations,
we must wait for them to return before returning to avoid writes to
resources which may have already been destructed.

See this blog post for more information:

http://blogs.msdn.com/b/oldnewthing/archive/2011/02/02/10123392.aspx

BUG=413494
TEST=manual inspect of cancellation states.

Review URL: https://codereview.chromium.org/570713003

Cr-Commit-Position: refs/heads/master@{#294946}