There would be no functional changes or performance impacts from
this fix, but this can avoid potential misuses and UBsan's bad-casting reports.

BUG=280225
R=dmazzoni@chromium.org,inferno@chromium.org

Review URL: https://codereview.chromium.org/361563003

git-svn-id: svn://svn.chromium.org/blink/trunk@177407 bbb929c8-8fbe-4397-9dbb-9b2b20218538