assert that we're not putting the same object twice into a dom wrapper map
assert that we're not putting the same object twice into a dom wrapper map

BUG=368095
R=vogelheim@chromium.org,dcarney@chromium.org,adamk@chromium.org

Review URL: https://codereview.chromium.org/317213004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@175688 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs