1. Really print WARN with default platform
2. Print UNREACHABLE, was getting lost because ~LogMessage was not reached

BUG=angleproject:1660

Change-Id: I56fad24895adfec8d81539c6628e65ec2f9e7722
Reviewed-on: https://chromium-review.googlesource.com/1021592Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>