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This is a reland of bd9b9ea0
Original change's description:
> Move WifiPollingPolicy to a global instance
> 
> The WifiPollingPolicy controls how frequently the network location
> provider will initiate Wi-Fi scans. Previously, the policy was attached
> to the WifiDataProvider, which is destroyed when there are no active
> geolocation API calls. Destruction of the provider causes us to lose track
> of our polling policy.
> 
> The policy allows the first scan to be performed immediately and schedules
> subsequent scans at regular intervals. When the provider is destroyed and
> recreated rapidly it considers each scan to be "first", which allows many
> Wi-Fi scans to be performed in a short window.
> 
> This change moves the policy state to a global instance so it can be saved
> when the provider is destroyed and recreated. The new policy allows the
> first-ever scan to be performed immediately, and subsequent scans may also
> be performed immediately if enough time has lapsed since the previous scan.
> In all other situations, the configured polling interval is enforced.
> 
> The global policy instance is leaked at shutdown.
> 
> BUG=764954
> 
> Change-Id: If16c73dfd7d6535346e86d0fab5df24f0c7b297d
> Reviewed-on: https://chromium-review.googlesource.com/850444
> Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
> Reviewed-by: Reilly Grant <reillyg@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#527219}

Bug: 764954
Change-Id: I394a671cdbcdb58d6e80c201777465d1d304987b
Reviewed-on: https://chromium-review.googlesource.com/860778Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528466}