Fix FrameOwnerPropertiesPropagationScrolling on 10.6 after r372906

ui/base/test/scoped_preferred_scroller_style_legacy_mac.mm

Index: ui/base/test/scoped_preferred_scroller_style_legacy_mac.mm
diff --git a/ui/base/test/scoped_preferred_scroller_style_legacy_mac.mm b/ui/base/test/scoped_preferred_scroller_style_legacy_mac.mm
index 2b01f9f98475f2fc5fb16204961907bc4ed4d75f..82e9a5b48423d58d11fc5723fcc30c46ad0a3b83 100644
--- a/ui/base/test/scoped_preferred_scroller_style_legacy_mac.mm
+++ b/ui/base/test/scoped_preferred_scroller_style_legacy_mac.mm
@@ -50,6 +50,9 @@ ScopedPreferredScrollerStyleLegacy::ScopedPreferredScrollerStyleLegacy() {
 }
 
 ScopedPreferredScrollerStyleLegacy::~ScopedPreferredScrollerStyleLegacy() {
+  if (!swizzler_)
+    return;  // Handle 10.6, which wouldn't have swizzled anything.
+
   swizzler_.reset();
 
   if ([NSScroller preferredScrollerStyle] != NSScrollerStyleLegacy)