merge to XFA: Fix -Wnon-virtual-dtor compiler warnings.

Original at URL: https://codereview.chromium.org/810883005

Note that the new code in XFA introduces many more of these, to be handled
separately.

TBR=brucedawson@chromium.org
TBR=thestig@chromium.org

BUG=https://code.google.com/p/pdfium/issues/detail?id=102

Review URL: https://codereview.chromium.org/842223004
27 files changed