[OSX] ConstrainedWindow with "proper" background color.

chrome/browser/ui/cocoa/constrained_window/constrained_window_custom_window.mm

 // Copyright (c) 2012 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 "chrome/browser/ui/cocoa/constrained_window/constrained_window_custom_window.h"
 
 #import "base/memory/scoped_nsobject.h"
 #import "chrome/browser/ui/chrome_style.h"
 #import "chrome/browser/ui/cocoa/constrained_window/constrained_window_sheet_controller.h"
 #include "skia/ext/skia_utils_mac.h"
+#include "ui/gfx/scoped_ns_graphics_context_save_gstate_mac.h"
 
 @implementation ConstrainedWindowCustomWindow
 
 - (id)initWithContentRect:(NSRect)contentRect {
   if ((self = [self initWithContentRect:contentRect
                               styleMask:NSBorderlessWindowMask
                                 backing:NSBackingStoreBuffered
                                   defer:NO])) {
     scoped_nsobject<NSView> contentView(
         [[ConstrainedWindowCustomWindowContentView alloc]
             initWithFrame:NSZeroRect]);
     [self setContentView:contentView];
   }
   return self;
 }
 
 - (id)initWithContentRect:(NSRect)contentRect
                 styleMask:(NSUInteger)windowStyle
                   backing:(NSBackingStoreType)bufferingType
                     defer:(BOOL)deferCreation {
   if ((self = [super initWithContentRect:contentRect
                                styleMask:NSBorderlessWindowMask
                                  backing:bufferingType
                                    defer:NO])) {
     [self setHasShadow:YES];
-    [self setBackgroundColor:[NSColor clearColor]];
+    [self setBackgroundColor:gfx::SkColorToCalibratedNSColor(
+        chrome_style::GetBackgroundColor())];
     [self setOpaque:NO];
     [self setReleasedWhenClosed:NO];
   }
   return self;
 }
 
 - (BOOL)canBecomeKeyWindow {
   return YES;
 }
 
@@ skipping 12 matching lines @@
                                   withWindowSize:windowContent.size];
   frame.size = windowContent.size;
   return frame;
 }
 
 @end
 
 @implementation ConstrainedWindowCustomWindowContentView
 
 - (void)drawRect:(NSRect)rect {
-  NSBezierPath* path = [NSBezierPath
+  gfx::ScopedNSGraphicsContextSaveGState state;
+
+  // Draw symmetric difference between rect path and oval path as "clear".
+  NSBezierPath* ovalPath = [NSBezierPath
       bezierPathWithRoundedRect:[self bounds]
                         xRadius:chrome_style::kBorderRadius
                         yRadius:chrome_style::kBorderRadius];
-  [gfx::SkColorToCalibratedNSColor(
-      chrome_style::GetBackgroundColor()) set];
+  NSBezierPath* path = [NSBezierPath bezierPathWithRect:[self bounds]];
+  [path appendBezierPath:ovalPath];
+  [path setWindingRule:NSEvenOddWindingRule];
+  [[NSGraphicsContext currentContext] setCompositingOperation:
+      NSCompositeCopy];
+  [[NSColor clearColor] set];
   [path fill];
 
   [[self window] invalidateShadow];
 }
 
 @end