Allow the track part to have negative margins so that it can slightly overlap buttons.  This
        is necessary to emulate the Aqua look in CSS.

        Reviewed by Sam Weinig

        * platform/ScrollbarThemeComposite.cpp:
        (WebCore::ScrollbarThemeComposite::paint):
        (WebCore::ScrollbarThemeComposite::hitTest):
        * rendering/RenderScrollbar.cpp:
        (WebCore::RenderScrollbar::setHoveredPart):
        (WebCore::RenderScrollbar::trackRect):
        * rendering/RenderScrollbar.h:
        * rendering/RenderScrollbarPart.cpp:
        (WebCore::RenderScrollbarPart::computeScrollbarWidth):
        (WebCore::RenderScrollbarPart::computeScrollbarHeight):
        * rendering/RenderScrollbarTheme.cpp:
        (WebCore::RenderScrollbarTheme::backButtonRect):
        (WebCore::RenderScrollbarTheme::forwardButtonRect):
        (WebCore::RenderScrollbarTheme::trackRect):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@37487 268f45cc-cd09-0410-ab3c-d52691b4dbfc