[Shadow DOM] Specifying scrollbar style of an element having RenderLayer in ShadowDOM does not work.
https://bugs.webkit.org/show_bug.cgi?id=107222

Reviewed by Simon Fraser.

Source/WebCore:

When specifying scrollbar style in ShadowDOM, it's not used. If the host element of ShadowDOM has scrollbar style,
it's used instead.

The root cause of this bug is that RenderLayer always see shadowAncestorNode's style. Some elements having
UserAgent ShadowDOM (e.g. <textarea>) should see host's style, but most of elements should see its style.

Test: fast/dom/shadow/scrollbar.html

* rendering/RenderLayer.cpp:
(WebCore::rendererForScrollbar):
(WebCore):
(WebCore::RenderLayer::createScrollbar):
(WebCore::RenderLayer::updateScrollCornerStyle):
(WebCore::RenderLayer::updateResizerStyle):

LayoutTests:

* fast/dom/shadow/scrollbar-expected.html: Added.
* fast/dom/shadow/scrollbar.html: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140287 268f45cc-cd09-0410-ab3c-d52691b4dbfc