MacViews: Add WidgetEventGenerator to abstract platform-specific event generation for tests.

ui/events/test/cocoa_test_event_utils.mm

Index: ui/events/test/cocoa_test_event_utils.mm
diff --git a/ui/events/test/cocoa_test_event_utils.mm b/ui/events/test/cocoa_test_event_utils.mm
index 4022104792941c943289f1a9986cbb83e8b1b09d..4f0cb67de0deaf7c190a7427875ab99785003e6b 100644
--- a/ui/events/test/cocoa_test_event_utils.mm
+++ b/ui/events/test/cocoa_test_event_utils.mm
@@ -10,6 +10,11 @@ ScopedClassSwizzler::ScopedClassSwizzler(Class target, Class source,
                                          SEL selector) {
   old_selector_impl_ = class_getInstanceMethod(target, selector);
   new_selector_impl_ = class_getInstanceMethod(source, selector);
+  if (!old_selector_impl_ && !new_selector_impl_) {
+    // Try class methods.
+    old_selector_impl_ = class_getClassMethod(target, selector);
+    new_selector_impl_ = class_getClassMethod(source, selector);
+  }
   method_exchangeImplementations(old_selector_impl_, new_selector_impl_);
 }