Revert 162775 "Expose Custom Elements to stable."

Speculative revert. This appears to kill polymer which we use
for browser tests on chromeos.

> Expose Custom Elements to stable.
> 
> BUG=180965
> 
> Review URL: https://codereview.chromium.org/90823002

TBR=dominicc@chromium.org

Review URL: https://codereview.chromium.org/94433003

git-svn-id: svn://svn.chromium.org/blink/trunk@162816 bbb929c8-8fbe-4397-9dbb-9b2b20218538