Web Inspector: detailed heap snapshot: Replace (Native objects) root element

Web Inspector: detailed heap snapshot: Replace (Native objects) root element
with '(Detached DOM trees)' and '(Document DOM trees)'
https://bugs.webkit.org/show_bug.cgi?id=77201

I think it'd be nice to replace one group containing all native objects with
separate groups for different types of native objects.

Reviewed by Yury Semikhatsky.

* bindings/v8/RetainedDOMInfo.cpp:
(WebCore::RetainedDOMInfo::GetGroupLabel):
(WebCore):
* bindings/v8/RetainedDOMInfo.h:
(RetainedDOMInfo):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@106115 268f45cc-cd09-0410-ab3c-d52691b4dbfc