Don't expose these on the Chromium (non-standalone) build. This fixes
some missing file warnings for the msvs-ninja build.

BUG=angle:928
BUG=459058

Change-Id: I8932c31223347358070955b33995a3e973c359b7
Reviewed-on: https://chromium-review.googlesource.com/253001Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: bratell at Opera <bratell@opera.com>
Reviewed-by: Kenneth Russell <kbr@chromium.org>