Revert of Remove prefixed Shadow DOM APIs in Element. (https://codereview.chromium.org/208503004/)

Reason for revert:
Breaks many browser tests: http://build.chromium.org/p/chromium.webkit/builders/Linux%20Tests/builds/35489

Original issue's description:
> Remove prefixed Shadow DOM APIs in Element.
> 
> It's time to remove. We've shipped unprefixed APIs for these APIs.
> 
> BUG=336121
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170008

TBR=dglazkov@chromium.org,esprehn@chromium.org,hayato@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=336121

Review URL: https://codereview.chromium.org/212303005

git-svn-id: svn://svn.chromium.org/blink/trunk@170036 bbb929c8-8fbe-4397-9dbb-9b2b20218538