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

ui/views/widget/native_widget_mac.mm

Index: ui/views/widget/native_widget_mac.mm
diff --git a/ui/views/widget/native_widget_mac.mm b/ui/views/widget/native_widget_mac.mm
index 729207a10ab0bdcb07da06e52f36acc7779517fc..ec0ca74b4d390afddbadb2d0972912fc9dd732ee 100644
--- a/ui/views/widget/native_widget_mac.mm
+++ b/ui/views/widget/native_widget_mac.mm
@@ -40,6 +40,7 @@ void NativeWidgetMac::InitNativeWidget(const Widget::InitParams& params) {
                                     backing:NSBackingStoreBuffered
                                       defer:NO]);
   bridge_->Init(window);
+  delegate_->OnNativeWidgetCreated(true);
 }
 
 NonClientFrameView* NativeWidgetMac::CreateNonClientFrameView() {
@@ -424,8 +425,7 @@ void NativeWidgetPrivate::ReparentNativeView(gfx::NativeView native_view,
 
 // static
 bool NativeWidgetPrivate::IsMouseButtonDown() {
-  NOTIMPLEMENTED();
-  return false;
+  return [NSEvent pressedMouseButtons] != 0;
 }
 
 // static