Mac: Fix SDK 10.11 build error from r424609.

chrome/browser/ui/cocoa/framed_browser_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/framed_browser_window.h"
 
 #include <math.h>
 #include <objc/runtime.h>
 #include <stddef.h>
 
@@ skipping 15 matching lines @@
 #import "ui/base/cocoa/nsview_additions.h"
 #include "ui/base/material_design/material_design_controller.h"
 
 // Implementer's note: Moving the window controls is tricky. When altering the
 // code, ensure that:
 // - accessibility hit testing works
 // - the accessibility hierarchy is correct
 // - close/min in the background don't bring the window forward
 // - rollover effects work correctly
 
+// The NSLayoutConstraint class hierarchy only exists in the 10.11 SDK. When
+// targeting something lower, constraintEqualToAnchor:constant: needs to be
+// invoked using duck typing.
+#if !defined(MAC_OS_X_VERSION_10_11) || \
+    MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_11
+@interface NSObject (NSLayoutConstraint)
+- (NSLayoutConstraint*)constraintEqualToAnchor:(id)anchor constant:(CGFloat)c;
+@end
+#endif
+
 namespace {
 
 // Size of the gradient. Empirically determined so that the gradient looks
 // like what the heuristic does when there are just a few tabs.
 const CGFloat kWindowGradientHeight = 24.0;
 
 }
 
 @interface FramedBrowserWindow (Private)
 
@@ skipping 212 matching lines @@
 
   // Do not use leadingAnchor because |ShouldDoExperimentalRTLLayout|
   // should determine if current locale is RTL.
   NSLayoutXAxisAnchor* leadingSourceAnchor = [button leftAnchor];
   NSLayoutXAxisAnchor* leadingTargetAnchor = [[button superview] leftAnchor];
   if (cocoa_l10n_util::ShouldDoExperimentalRTLLayout()) {
     leadingSourceAnchor = [button rightAnchor];
     leadingTargetAnchor = [[button superview] rightAnchor];
     leadingOffset = -leadingOffset;
   }
-  [[leadingSourceAnchor constraintEqualToAnchor:leadingTargetAnchor
-                                       constant:leadingOffset] setActive:YES];
+
+#if !defined(MAC_OS_X_VERSION_10_11) || \
+    MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_11
+  id leadingSourceAnchorDuck = leadingSourceAnchor;
+#else
+  NSLayoutXAxisAnchor* leadingSourceAnchorDuck = leadingSourceAnchor;
+#endif
+  [[leadingSourceAnchorDuck constraintEqualToAnchor:leadingTargetAnchor
+                                           constant:leadingOffset]
+      setActive:YES];
 }
 
 - (void)adjustCloseButton:(NSNotification*)notification {
   [self adjustButton:[notification object]
               ofKind:NSWindowCloseButton];
 }
 
 - (void)adjustMiniaturizeButton:(NSNotification*)notification {
   [self adjustButton:[notification object]
               ofKind:NSWindowMiniaturizeButton];
@@ skipping 218 matching lines @@
   [self childWindowsDidChange];
 }
 
 - (void)childWindowsDidChange {
   id delegate = [self delegate];
   if ([delegate respondsToSelector:@selector(childWindowsDidChange)])
     [delegate childWindowsDidChange];
 }
 
 @end