MacViews: Implement NativeWidgetMac::SetOpacity function.

ui/views/widget/native_widget_mac_unittest.mm

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ui/views/widget/native_widget_mac.h"
 
 #import <Cocoa/Cocoa.h>
 
 #import "base/mac/foundation_util.h"
 #import "base/mac/scoped_nsobject.h"
@@ skipping 1071 matching lines @@
   NSRect expected = [[[NSScreen screens] firstObject] visibleFrame];
   NSRect actual = gfx::ScreenRectToNSRect(widget.GetWorkAreaBoundsInScreen());
   EXPECT_FALSE(NSIsEmptyRect(actual));
   EXPECT_NSEQ(expected, actual);
 
   [widget.GetNativeWindow() close];
   actual = gfx::ScreenRectToNSRect(widget.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
 
 @implementation TestStopAnimationWaiter
 - (void)setWindowStateForEnd {
   views::test::ScopedSwizzleWaiter::GetMethodAndMarkCalled()(self, _cmd);
 }
 @end
 
 @implementation NativeWidgetMacTestWindow
 
 @synthesize invalidateShadowCount = invalidateShadowCount_;
 
 - (void)invalidateShadow {
   ++invalidateShadowCount_;
   [super invalidateShadow];
 }
 
 @end