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CHECK() produces crashes and sometimes these are prevalent enough
for crash triagers to file and ping crbugs about, which just wastes
everyone's time. This changes the CHECKs to a printed message via
puts and an exit() call.

Differentiates between invalid variation ids coming from command
line vs. those from about:flags and uses a NOTREACHED in the latter
case.

Also, cleans up the code a bit by changing the APIs around passing
additional variation ids - making the param const and making a
helper function private. Also fixes some lint errors around includes
and header guards.

BUG=812164
TBR=sky@chromium.org,rohitrao@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Id765738791ada14540ff51fe5f6fc837dd42e7c7
Reviewed-on: https://chromium-review.googlesource.com/919194Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537516}