Use SPECIALIZE_TYPE_TRAITS_*() macro for RenderScrollbar
https://bugs.webkit.org/show_bug.cgi?id=137854

Reviewed by Darin Adler.

Use SPECIALIZE_TYPE_TRAITS_*() macro for RenderScrollbar instead of a
custom toRenderScrollbar() function, for consistency.

No new tests, no behavior change.

* rendering/RenderScrollbar.h:
(isType):
(WebCore::toRenderScrollbar): Deleted.
* rendering/RenderScrollbarTheme.cpp:
(WebCore::RenderScrollbarTheme::minimumThumbLength):
(WebCore::RenderScrollbarTheme::backButtonRect):
(WebCore::RenderScrollbarTheme::forwardButtonRect):
(WebCore::RenderScrollbarTheme::trackRect):
(WebCore::RenderScrollbarTheme::constrainTrackRectToTrackPieces):
(WebCore::RenderScrollbarTheme::willPaintScrollbar):
(WebCore::RenderScrollbarTheme::didPaintScrollbar):
(WebCore::RenderScrollbarTheme::paintScrollbarBackground):
(WebCore::RenderScrollbarTheme::paintTrackBackground):
(WebCore::RenderScrollbarTheme::paintTrackPiece):
(WebCore::RenderScrollbarTheme::paintButton):
(WebCore::RenderScrollbarTheme::paintThumb):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@174874 268f45cc-cd09-0410-ab3c-d52691b4dbfc