Add a release assert in Element's constructor to track down a crasher

R=abarth@chromium.org
BUG=312410

Review URL: https://codereview.chromium.org/102993010

git-svn-id: svn://svn.chromium.org/blink/trunk@164442 bbb929c8-8fbe-4397-9dbb-9b2b20218538