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: svn://svn.chromium.org/blink/trunk@175688 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Cleanup: Remove unused include headers and forward declarations

BUG=None

Review URL: https://codereview.chromium.org/311263009

git-svn-id: svn://svn.chromium.org/blink/trunk@175689 bbb929c8-8fbe-4397-9dbb-9b2b20218538