Clean up ValidateOutputs Remove the validateOutputs method from Compiler and replace it with a static method alongside the traverser. This encapsulates the ValidateOutputs implementation better. TEST=angle_unittests BUG=angleproject:2068 Change-Id: I1788cb9726db41ca35fd0e746f8d48ced7fee74f Reviewed-on: https://chromium-review.googlesource.com/535477 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>