[Mac] Avoid "adding unknown subview" warning.

chrome/browser/ui/cocoa/browser_window_layout.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 "chrome/browser/ui/cocoa/browser_window_layout.h"
 
 #include <math.h>
 #include <string.h>
 
 #include "base/logging.h"
 #include "base/mac/mac_util.h"
 #include "chrome/browser/ui/cocoa/l10n_util.h"
 #import "chrome/browser/ui/cocoa/tabs/tab_strip_controller.h"
 
 namespace chrome {
 
 // The height of the tab strip.
 const CGFloat kTabStripHeight = 37;
 
+bool ShouldUseFullSizeContentView() {
+  // Prior to macOS 10.11 Chrome has added a subview above the window's content
+  // view, which the Appkit warns about at runtime. This was done to make sure
+  // that window buttons are always displayed above the content. Presumably,
+  // doing so may break in a future macOS release. Using
+  // NSFullSizeContentViewWindowMask makes window buttons displayed inside
+  // the titlebar, so they are not covered by the content view.
+  return base::mac::IsAtLeastOS10_11();
+}
+
 }  // namespace chrome
 
 namespace {
 
 // Insets for the location bar, used when the full toolbar is hidden.
 // TODO(viettrungluu): We can argue about the "correct" insetting; I like the
 // following best, though arguably 0 inset is better/more correct.
 const CGFloat kLocBarLeftRightInset = 1;
 const CGFloat kLocBarTopInset = 0;
 const CGFloat kLocBarBottomInset = 1;
@@ skipping 364 matching lines @@
 
 @end
 
 @implementation BrowserWindowLayout (ExposedForTesting)
 
 - (void)setOSYosemiteOrLater:(BOOL)osYosemiteOrLater {
   parameters_.isOSYosemiteOrLater = osYosemiteOrLater;
 }
 
 @end