Revert of Re-landing r170008, removing prefixed Shadow DOM APIs in Element. (https://codereview.chromium.org/218943003/)

Reason for revert:
Broke OpenAudioFiles/FileManagerBrowserTest again.

http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%282%29/builds/22744


Original issue's description:
> 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
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170460

TBR=dglazkov@chromium.org,esprehn@chromium.org,hayato@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=242153

Review URL: https://codereview.chromium.org/220123005

git-svn-id: svn://svn.chromium.org/blink/trunk@170519 bbb929c8-8fbe-4397-9dbb-9b2b20218538