Third attempt to remove prefixed Shadow DOM ApIs in Element.

The first attempt: https://codereview.chromium.org/208503004
The second attempt: https://codereview.chromium.org/218943003

I've fixed the causes of the previous reverts as follows:

- https://codereview.chromium.org/214093002/
- https://codereview.chromium.org/217423021/

This time should be successful, hopefully.

TBR=dglazkov@chromium.org,esprehn@chromium.org
BUG=242153

Review URL: https://codereview.chromium.org/227203004

git-svn-id: svn://svn.chromium.org/blink/trunk@170949 bbb929c8-8fbe-4397-9dbb-9b2b20218538