Add a ShadowRoot constructor as 'WebKitShadowRootConstructor', enabled by SHADOW_DOM...

Add a ShadowRoot constructor as 'WebKitShadowRootConstructor', enabled by SHADOW_DOM flag.
https://bugs.webkit.org/show_bug.cgi?id=76354

Reviewed by Hajime Morita.

We use vendor-prefixed name, 'WebKitShadowRoot', instead of 'ShadowRoot'
since this is a feature under development.

Source/WebCore:

* dom/ShadowRoot.cpp:
(WebCore::ShadowRoot::create):
(WebCore):
* dom/ShadowRoot.h:
(WebCore):
(ShadowRoot):
* dom/ShadowRoot.idl:
* page/DOMWindow.idl:

LayoutTests:

* fast/dom/shadow/shadow-root-js-api.html:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106208 268f45cc-cd09-0410-ab3c-d52691b4dbfc