Fix build warnings about unused private variables on clang BUG=angle:638,677 Change-Id: Id9e80f0b86d9c24633b2d353a409844c22441d21 Reviewed-on: https://chromium-review.googlesource.com/203429 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Shannon Woods <shannonwoods@chromium.org>