[Shadow DOM] Extend a 'path' notation in utility function so that we can specify...

[Shadow DOM] Extend a 'path' notation in utility function so that we can specify arbitrary ShadowRoot.
https://bugs.webkit.org/show_bug.cgi?id=80405

Reviewed by Dimitri Glazkov.

Extract common utility function, getElementByIdInShadow(path), into a shadow-dom.js
(renamed from create-dom.js) and support 'consecutive slashes' notation so that
we can specify arbitrary ShadowRoot in shadow DOM tree.  Until now, we can get an element
only from the youngest ShadowRoot.

* fast/dom/shadow/access-key.html:
* fast/dom/shadow/get-element-by-id-in-shadow-root-expected.txt:
* fast/dom/shadow/get-element-by-id-in-shadow-root.html:
* fast/dom/shadow/resources/shadow-dom.js: Renamed from LayoutTests/fast/dom/shadow/resources/create-dom.js.
(createShadowRoot):
(createDOM):
(isShadowRoot):
(getElementInShadowTreeStack):
* fast/dom/shadow/shadow-boundary-events.html:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@110022 268f45cc-cd09-0410-ab3c-d52691b4dbfc