GL_MAX_SERVER_WAIT_TIMEOUT is only allowed to be positive, ensure that with
a std::max.

BUG=angleproject:2961

Change-Id: I8dd66df24d35c8ae81038d6d9cd0bad0d51f1a80
Reviewed-on: https://chromium-review.googlesource.com/c/1335889Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>