[Mac] Fix Toolbar glimpse during fullscreen transition

chrome/browser/ui/cocoa/browser_window_controller.mm

 // Copyright 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/browser_window_controller.h"
 
 #include <cmath>
 #include <numeric>
 #include <utility>
 
@@ skipping 2044 matching lines @@
 }
 
 - (BOOL)isTabbedWindow {
   return browser_->is_type_tabbed();
 }
 
 - (NSRect)savedRegularWindowFrame {
   return savedRegularWindowFrame_;
 }
 
+- (BOOL)isFullscreenTransitionInProgress {
+  return enteringAppKitFullscreen_ || exitingAppKitFullscreen_;
+}
+
 @end  // @implementation BrowserWindowController(WindowType)