Turn down warning level for GN build of image_diff utility

Next attempt to get GN build to work.  Once this lands, doing a DEPS
roll against the win8_chromium_gn_dbg trybot seems to catch the issue.

Restore image_diff_png.cc to its previous state.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1024243003
2 files changed