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The component updater can look into the BITS queue to see if
a job already exist to download a given component. This stat
provides an insight of whether this optimization is useful.

TBR=waffles

Bug: 737730
Change-Id: I56b31ad7b382197970b2dc49e8b42a7620a8e943
Reviewed-on: https://chromium-review.googlesource.com/564178Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485352}