Reapply r162775 to expose Custom Elements to stable.

This reverts r162816.

Chrome OS virtual keyboard has implemented a workaround in r237854 so
that virtual keyboard continues to use the Polymer Custom Elements
polyfill in the interim until Polymer updates their polyfill to
feature detect Custom Elements and Shadow DOM separately.

BUG=180965
TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/96023002

git-svn-id: svn://svn.chromium.org/blink/trunk@162866 bbb929c8-8fbe-4397-9dbb-9b2b20218538