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CompletionHandler is used in CacheThread_BlockFile.
CacheThread_BlockFile is lazy + leaky instance, thus, there was race
 condition when all lazy instances are already destroyed, but message
 io pump still processes some io overlapped operation,leading to using
 IOHandler after free.
Make completion handler to be refcounted and add ref to each overlapped
 operation it participates with.

R=jkarlin@chromium.org, morlovich@chromium.org

Bug: 
Change-Id: I7bd5fbdc83aae05489d5089ee16a94857ea1b104
Reviewed-on: https://chromium-review.googlesource.com/793158Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Commit-Queue: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523556}