Re-landing r170008, removing prefixed Shadow DOM APIs in Element.

The previous attempt, https://codereview.chromium.org/208503004, was reverted in https://codereview.chromium.org/212303005/.
The reason of reverting is that there is a JavaScript code uses prefixed Shadow DOM API in the chromium side.
I've fixed that in https://codereview.chromium.org/214093002/.

Let me re-land r170008. I hope it won't break anything this time.

BUG=242153

Review URL: https://codereview.chromium.org/218943003

git-svn-id: svn://svn.chromium.org/blink/trunk@170460 bbb929c8-8fbe-4397-9dbb-9b2b20218538