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When the network location provider is in use, Chrome periodically
scans for nearby WiFi access points. The MAC addresses of these
access points are sent to a Google service, which returns a position
estimate. To limit network traffic when a device is not moving, a
cache of recent position estimates is maintained. If the set of MAC
addresses exactly matches a previous query, the cached estimate is
used instead.

This adds metrics for recording whether a cached value was used. It
also records how many items were in the cache when the cache is
queried.

BUG=944320

Change-Id: I385e1f76099207dd9276edc258a958959945058f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1532255
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650536}