Make Node::showTreeForThis (and Node::showTreeForThisAcrossFrame) dump multiple shado...

Make Node::showTreeForThis (and Node::showTreeForThisAcrossFrame) dump multiple shadow roots.
https://bugs.webkit.org/show_bug.cgi?id=79351

Reviewed by Dimitri Glazkov.

No new tests since these utility functions are only available in debug build.
I manually tested in GDB session.

* dom/Node.cpp:
(WebCore::oldestShadowRootFor):
(WebCore::traverseTreeAndMark):
(WebCore::parentOrHostOrFrameOwner):
(WebCore::showSubTreeAcrossFrame):
* dom/ShadowRoot.h:
(WebCore::ShadowRoot::youngerShadowRoot):
(WebCore::ShadowRoot::olderShadowRoot):
(WebCore::toShadowRoot):
(WebCore):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108770 268f45cc-cd09-0410-ab3c-d52691b4dbfc