[Mac] Avoid "adding unknown subview" warning.

chrome/browser/ui/cocoa/browser_window_layout.mm

Index: chrome/browser/ui/cocoa/browser_window_layout.mm
diff --git a/chrome/browser/ui/cocoa/browser_window_layout.mm b/chrome/browser/ui/cocoa/browser_window_layout.mm
index 5afd82c0ce3297fa07cc9407a03b4620eedb37cb..153a8d5610b1de49bcea3c88e141e6c34859fcfc 100644
--- a/chrome/browser/ui/cocoa/browser_window_layout.mm
+++ b/chrome/browser/ui/cocoa/browser_window_layout.mm
@@ -17,6 +17,16 @@ 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 {