Add keyboard accessibility to ConstrainedWindowButton

chrome/browser/ui/cocoa/constrained_window/constrained_window_button.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_button.h"
 
 #include "base/mac/scoped_nsobject.h"
 #import "chrome/browser/ui/cocoa/key_equivalent_constants.h"
 #include "skia/ext/skia_utils_mac.h"
 #import "third_party/molokocacao/NSBezierPath+MCAdditions.h"
@@ skipping 227 matching lines @@
 + (void)DrawBorderForPath:(NSBezierPath*)path
                  withCell:(id<ConstrainedWindowButtonDrawableCell>)cell
                    inView:(NSView*)view {
   if ([[[view window] firstResponder] isEqual:view])
     [[NSColor colorWithCalibratedRed:0.30 green:0.57 blue:1.0 alpha:1.0] set];
   else
     [GetButtonBorderColor(cellButtonState(cell)) set];
   [path stroke];
 }
 
+- (BOOL)canBecomeKeyView {
+  return YES;
+}
+
 @end
 
 @implementation ConstrainedWindowButtonCell
 
 @synthesize isMouseInside = isMouseInside_;
 
 - (void)drawBezelWithFrame:(NSRect)frame inView:(NSView*)controlView {
   // Inset to leave room for shadow.
   --frame.size.height;
 
@@ skipping 23 matching lines @@
   size.width += kButtonPaddingX * 2;
   return size;
 }
 
 - (NSAttributedString*)getAttributedTitle {
   return GetButtonAttributedString(
       [self title], [self keyEquivalent], self);
 }
 
 @end