dm: fix unused-private-field error when skia_use_expat=false

Change-Id: I309aa420a7900c048c34873ede618f123719823c
Reviewed-on: https://skia-review.googlesource.com/115928
Commit-Queue: Hal Canary <halcanary@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
1 file changed