Revert of Reland removing prefixed Shadow DOM APIs in Element. (https://codereview.chromium.org/216973003/)

Reason for revert:
This broke the following tests in browser_tests on Linux ChromiumOS Tests bots:

- OpenAudioFiles/FileManagerBrowserTest.Test/0
- OpenAudioFiles/FileManagerBrowserTest.Test/1
- OpenAudioFiles/FileManagerBrowserTest.Test/2

http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%282%29/builds/22698


Original issue's description:
> Reland removing prefixed Shadow DOM APIs in Element.
> 
> Now that https://codereview.chromium.org/214093002/ has landed,
> we can reland this safely.
> 
> BUG=336121
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170374

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

Review URL: https://codereview.chromium.org/218653002

git-svn-id: svn://svn.chromium.org/blink/trunk@170422 bbb929c8-8fbe-4397-9dbb-9b2b20218538