Make scrollAnimatorEnabled work on Mac like it does on other platforms

base/mac/mac_util.h

Index: base/mac/mac_util.h
diff --git a/base/mac/mac_util.h b/base/mac/mac_util.h
index 7772e883fad707e9790dfe7200d40bc02463c564..66ed06f8bdd489ff56f28d98dc6d262cb47a8fec 100644
--- a/base/mac/mac_util.h
+++ b/base/mac/mac_util.h
@@ -244,6 +244,10 @@ BASE_EXPORT bool ParseModelIdentifier(const std::string& ident,
                                       int32_t* major,
                                       int32_t* minor);
 
+// Get the system setting determining whether scroll animations (e.g. when
+// user uses home/end keys) should be enabled.
+BASE_EXPORT bool IsScrollAnimationEnabled();
+
 }  // namespace mac
 }  // namespace base