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This is a reland of b58c0181

The original CL used an unset class variable rather than the callback
being passed into the function.

The reland solves this by using the correct function variable and adds
better unittesting to confirm the correct behavior.

Original change's description:
> [Previews] Add offline check before HintsFetch.
>
> Only attempt to fetch hints if the network is available.
>
> Note: The Optimization Guide does not support iOS but is in components
> due to supporting Previews. A future refactor of Previews may allow for
>  moving Optimization Guide to the browser layer. An assertion was added
> to the BUILD.gn file to make the iOS decision more clear.
>
> Bug: 986817
> Change-Id: Iafa4101b42147530e0b38f2d6a4eb36a7ddac37c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715505
> Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
> Reviewed-by: Tarun Bansal <tbansal@chromium.org>
> Reviewed-by: Robert Ogden <robertogden@chromium.org>
> Commit-Queue: Michael Crouse <mcrouse@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#681180}

TBR=webrtc-chromium-sheriffs-robots@google.com,kinuko@google.com

Bug: 986817
Change-Id: I4a2e51c8eb7b498b3d9cb542da265e5ebc15431f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1729053
Commit-Queue: Michael Crouse <mcrouse@chromium.org>
Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683272}