Compile-time assert that we do not memset things with vtables to zero

R=ager@chromium.org, haraken@chromium.org, mikhail.pozdnyakov@intel.com
BUG=

Review URL: https://codereview.chromium.org/295633002

git-svn-id: svn://svn.chromium.org/blink/trunk@174288 bbb929c8-8fbe-4397-9dbb-9b2b20218538