clang/win: Fix -Wunused-private-field warnings in debug builds. No intended behavior change. BUG=505317 Change-Id: I5dfa1d67715b18133f3373ca00a6d1d96b678043 Reviewed-on: https://chromium-review.googlesource.com/284850 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Nico Weber <thakis@chromium.org>