Macros like ASSERT and UNREACHABLE call gl::trace, which at some
point we might want to use to call the angle Platform logging code.
Standalone tests won't easily have access to the platform, so for
now just log errors in other ways.

This also corrects some logic in GLSLTest which was calling
internal methods in libANGLE when it didn't have to.

BUG=angleproject:1660

Change-Id: Idecbd97f2de7916b35bd78f5b7cd02b156ea100d
Reviewed-on: https://chromium-review.googlesource.com/419134
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>