https://bugs.webkit.org/show_bug.cgi?id=91564

Reviewed by Eric Carlson.

We introduce Element::userAgentShadowRoot() to return a UserAgentShadowRoot.
It returns the oldest shadow root if any, and checks the type of ShadowRoot in debug build.

Previously, we have used elementShadow()->oldestShadowRoot() to get UserAgentShadowRoot.
The class implementor will know the oldest shadow root is UserAgentShadowRoot, but it is not
clear for the other people. This method will make the code intention clearer.

We will introduce userAgentShadowRoot to the other elements (e.g. HTMLInputElement) after this patch.

No new tests, simple refactoring.

* dom/Element.cpp:
(WebCore::Element::userAgentShadowRoot):
(WebCore):
* dom/Element.h:
(Element):
* html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::createShadowSubtree):
(WebCore::HTMLMediaElement::willAddAuthorShadowRoot):
(WebCore::HTMLMediaElement::mediaControls):
(WebCore::HTMLMediaElement::hasMediaControls):
(WebCore::HTMLMediaElement::createMediaControls):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@123071 268f45cc-cd09-0410-ab3c-d52691b4dbfc