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..623ec8647403db850cfbb262c1f9a89cee1947b9 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();
+
+  auto old_opacity = [ns_window alphaValue];

[tapted, 2016/01/18 00:35:14]

nit: auto->CGFloat (auto is allowed "when it helps readability" which doesn't
really apply here)

[Andrey Kraynov, 2016/01/18 09:43:54]

Done.

+  widget->SetOpacity(0xAA);
+  EXPECT_NE(old_opacity, [ns_window alphaValue]);
+  EXPECT_EQ(0xAA / 255.0, [ns_window alphaValue]);

[tapted, 2016/01/18 00:35:14]

nit: EXPECT_DOUBLE_EQ

(or try EXPECT_FLOAT_EQ if that fails, but I think that would only be on 32-bit
which is no longer ever true for Mac)

[Andrey Kraynov, 2016/01/18 09:43:54]

Done.

+
+  widget->CloseNow();
+}
+
 }  // namespace test
 }  // namespace views