Don't expose internal variables in the shader translator interface.
This affects the ViewID_OVR varying needed for instanced multiview,
which is so far the only variable of this kind.

This fixes the translator trying to add initialization for internal
variables in initializeOutputVariables. Since they are variables added
by ANGLE, they should never need extra initialization.

BUG=angleproject:2112
TEST=angle_unittests

Change-Id: I93ee2956c8180053806ce450d93f162f78a45d8f
Reviewed-on: https://chromium-review.googlesource.com/579050Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>