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Currently compilation fails because there's an iOS 11 only constant
in a switch-case, so there's a warning -Wunguarded-availability-new.
This silences the warning when compiling this case.

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I3fe6bc7e66a16c3a8590af3472f345f8a883e452
Reviewed-on: https://chromium-review.googlesource.com/774858Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517101}