Having UNIMPLEMENTED be an error was problematic in a few places. Some
tests would pass despite hitting the macro. We were relying on this in
a bunch of places. Making it a warning is an easier solution than
fixing all of the cases.

Going forward we can fix the warnings and make warnings produce test
failures. This is similar to enabling warnings as errors in C++ code.

Bug: angleproject:2552
Change-Id: Iaf0479cd6bd6a9e85aaccbbee13c322166ff7b5a
Reviewed-on: https://chromium-review.googlesource.com/1064724
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>