MacViews: Implement NativeWidgetMac::SetOpacity function.

ui/views/widget/native_widget_mac_unittest.mm

Index: ui/views/widget/native_widget_mac_unittest.mm
diff --git a/ui/views/widget/native_widget_mac_unittest.mm b/ui/views/widget/native_widget_mac_unittest.mm
index 5a48f0257223f75064732e12b1a76061c18a7cd2..b8d29dde228b029abd0719f5eba9d48c4071abcd 100644
--- a/ui/views/widget/native_widget_mac_unittest.mm
+++ b/ui/views/widget/native_widget_mac_unittest.mm
@@ -1089,6 +1089,19 @@ TEST_F(NativeWidgetMacTest, GetWorkAreaBoundsInScreen) {
   EXPECT_TRUE(NSIsEmptyRect(actual));
 }
 
+// Test that Widget opacity can be changed.
+TEST_F(NativeWidgetMacTest, ChangeOpacity) {
+  Widget* widget = CreateTopLevelPlatformWidget();
+  NSWindow* ns_window = widget->GetNativeWindow();
+
+  CGFloat old_opacity = [ns_window alphaValue];
+  widget->SetOpacity(0xAA);
+  EXPECT_NE(old_opacity, [ns_window alphaValue]);
+  EXPECT_DOUBLE_EQ(0xAA / 255.0, [ns_window alphaValue]);
+
+  widget->CloseNow();
+}
+
 }  // namespace test
 }  // namespace views