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..43d411dc6107680f8cb3ded63b206855f0041966 100644
--- a/Source/core/platform/mac/ScrollAnimatorMac.mm
+++ b/Source/core/platform/mac/ScrollAnimatorMac.mm
@@ -63,15 +63,16 @@ static bool supportsContentAreaScrolledInDirection()
     return globalSupportsContentAreaScrolledInDirection;
 }
 
-static ScrollbarThemeMac* macScrollbarTheme()
+static ScrollbarThemeMacOverlayAPI* macOverlayScrollbarTheme()
 {
+    RELEASE_ASSERT_WITH_SECURITY_IMPLICATION(isScrollbarOverlayAPIAvailable());
     ScrollbarTheme* scrollbarTheme = ScrollbarTheme::theme();
-    return !scrollbarTheme->isMockTheme() ? static_cast<ScrollbarThemeMac*>(scrollbarTheme) : 0;
+    return !scrollbarTheme->isMockTheme() ? static_cast<ScrollbarThemeMacOverlayAPI*>(scrollbarTheme) : 0;
 }
 
 static ScrollbarPainter scrollbarPainterForScrollbar(Scrollbar* scrollbar)
 {
-    if (ScrollbarThemeMac* scrollbarTheme = macScrollbarTheme())
+    if (ScrollbarThemeMacOverlayAPI* scrollbarTheme = macOverlayScrollbarTheme())
         return scrollbarTheme->painterForScrollbar(scrollbar);
 
     return nil;
@@ -1189,7 +1190,7 @@ void ScrollAnimatorMac::updateScrollerStyle()
         return;
     }
 
-    ScrollbarThemeMac* macTheme = macScrollbarTheme();
+    ScrollbarThemeMacOverlayAPI* macTheme = macOverlayScrollbarTheme();
     if (!macTheme) {
         m_needsScrollerStyleUpdate = false;
         return;