Reland "Variations service now checks explicitely if previous attempt was HTTP."

This reverts commit 8d41f8c9c5f71f94133e1e557701521b5e464c63.

Reason for revert: This CL reverts the revert, and fixes the flakiness in the test by allowing one or two requests in the test.

Bug: 904108, 902727

Original change's description:
> Revert "Variations service now checks explicitely if previous attempt was HTTP."
>
> This reverts commit 91d246885ead6155c131e027d99be1aa79b9ae3c.
>
> Reason for revert: This CL made ChromeBrowserMainBrowserTest.VariationsServiceStartsRequestOnNetworkChange flaky (crbug.com/904108) reverting while investigating why.
>
> Original change's description:
> > Variations service now checks explicitely if previous attempt was HTTP.
> >
> > Variations service now sets a flag if the last request was an HTTP
> > retry, and uses it to decide whether to retry again, instead of
> > relying on the scheme of the final URL. Also, retries are now disabled
> > if the fallback url is HTTPS.
> >
> > Bug: 902727
> > Change-Id: Ibd6df4a22bc6302b231aff0ae32f8e1c8a1f277c
> > Reviewed-on: https://chromium-review.googlesource.com/c/1325035
> > Commit-Queue: Carlos IL <carlosil@chromium.org>
> > Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#607022}
>
> TBR=asvitkine@chromium.org,carlosil@chromium.org
>
> # Not skipping CQ checks because original CL landed > 1 day ago.
>
> Bug: 902727
> Change-Id: I67036ea20097f889e71de5179a2e4510a201388d
> Reviewed-on: https://chromium-review.googlesource.com/c/1334421
> Reviewed-by: Carlos IL <carlosil@chromium.org>
> Commit-Queue: Carlos IL <carlosil@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#607882}

Change-Id: I5618dd7c48849ddf1fc8cd8f613ba7d83ebc66c0
Reviewed-on: https://chromium-review.googlesource.com/c/1335666
Commit-Queue: Carlos IL <carlosil@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611152}
4 files changed