Revert "Fix compilation error with -Werror=delete-non-virtual-dtor flag set"

It turns out my checkout was probably outdated at the time I got this warning,
as GCC 4.8 does not complain about this anymore.

This reverts commit 01c4780c6ecd9901f965ded475e8f9b638738921.

Conflicts:
runtime/browser/xwalk_browser_main_parts.cc