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>