Add support for both positive and negative margins to tracks and track pieces.

        Reviewed by Tim Hatcher

        * platform/ScrollbarThemeComposite.cpp:
        (WebCore::ScrollbarThemeComposite::paint):
        (WebCore::ScrollbarThemeComposite::hitTest):
        (WebCore::ScrollbarThemeComposite::splitTrack):
        (WebCore::ScrollbarThemeComposite::trackPieceRects):
        (WebCore::ScrollbarThemeComposite::trackPosition):
        (WebCore::ScrollbarThemeComposite::trackLength):
        * platform/ScrollbarThemeComposite.h:
        (WebCore::ScrollbarThemeComposite::adjustTrackPieceRect):
        * rendering/RenderScrollbar.cpp:
        (WebCore::RenderScrollbar::trackRect):
        (WebCore::RenderScrollbar::trackPieceRectWithMargins):
        * rendering/RenderScrollbar.h:
        * rendering/RenderScrollbarPart.cpp:
        (WebCore::RenderScrollbarPart::computeScrollbarWidth):
        (WebCore::RenderScrollbarPart::computeScrollbarHeight):
        * rendering/RenderScrollbarTheme.cpp:
        (WebCore::RenderScrollbarTheme::adjustTrackPieceRect):
        * rendering/RenderScrollbarTheme.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@37507 268f45cc-cd09-0410-ab3c-d52691b4dbfc