Turn on -Werror for standalone builds.
Turn on -Werror for standalone builds.

This required suppressing the non-virtual-dtor warning, since
ICU isn't clean.  Getting -Werror in exchange for that seems
worthwhile.

Chromium's builds are controlled by its own files, which already
-Werror.

R=ochang@chromium.org

Review URL: https://codereview.chromium.org/1765893003 .

DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium