Split ScrollbarThemeMac into Overlay and NonOverlay subclasses

Source/core/platform/mac/ScrollAnimatorMac.mm

Index: Source/core/platform/mac/ScrollAnimatorMac.mm
diff --git a/Source/core/platform/mac/ScrollAnimatorMac.mm b/Source/core/platform/mac/ScrollAnimatorMac.mm
index 51624ec0c4bea4842af3d920d197556dab32a673..28de722fc3c87c9b5faa743f04c6d38bec340247 100644
--- a/Source/core/platform/mac/ScrollAnimatorMac.mm
+++ b/Source/core/platform/mac/ScrollAnimatorMac.mm
@@ -63,15 +63,15 @@ static bool supportsContentAreaScrolledInDirection()
     return globalSupportsContentAreaScrolledInDirection;
 }
 
-static ScrollbarThemeMac* macScrollbarTheme()
+static ScrollbarThemeMacOverlay* macOverlayScrollbarTheme()
 {
     ScrollbarTheme* scrollbarTheme = ScrollbarTheme::theme();
-    return !scrollbarTheme->isMockTheme() ? static_cast<ScrollbarThemeMac*>(scrollbarTheme) : 0;
+    return !scrollbarTheme->isMockTheme() ? static_cast<ScrollbarThemeMacOverlay*>(scrollbarTheme) : 0;

[enne (OOO), 2013/07/31 18:23:59]

So, I *think* this is only for overlay scrollbars, but am not 100% sure.  Do you
have more confidence that this is the case?

Should there be some ASSERT here that the overlay API is supported?

[ccameron, 2013/07/31 20:26:57]

Yes, there should be an ASSERT added. Done.

 }
 
 static ScrollbarPainter scrollbarPainterForScrollbar(Scrollbar* scrollbar)
 {
-    if (ScrollbarThemeMac* scrollbarTheme = macScrollbarTheme())
+    if (ScrollbarThemeMacOverlay* scrollbarTheme = macOverlayScrollbarTheme())
         return scrollbarTheme->painterForScrollbar(scrollbar);
 
     return nil;
@@ -1189,7 +1189,7 @@ void ScrollAnimatorMac::updateScrollerStyle()
         return;
     }
 
-    ScrollbarThemeMac* macTheme = macScrollbarTheme();
+    ScrollbarThemeMacOverlay* macTheme = macOverlayScrollbarTheme();
     if (!macTheme) {
         m_needsScrollerStyleUpdate = false;
         return;