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Use SystemCookieStoreUtil to provide the requestContextGetter with the correct
base cookie store (either NSHTTPCookieStore or WKHTTPCookieStore).
This means that when WKHTTPCookieStore feature flag is enabled and it's iOS11+
 cookie store will be using WKHTTPCookieStore.

Bug: 779106,759229
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I97695aef8c75d88861d0694c2cf4983447bfe128
Reviewed-on: https://chromium-review.googlesource.com/809744Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Mohammad Refaat <mrefaat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523921}