Force constrained windows from nibs to be borderless

chrome/browser/ui/cocoa/constrained_window/constrained_window_custom_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/constrained_window/constrained_window_custom_window.h"
 
 #import "base/memory/scoped_nsobject.h"
 #import "chrome/browser/ui/cocoa/constrained_window/constrained_window_sheet_controller.h"
 #import "chrome/browser/ui/constrained_window.h"
 #import "chrome/browser/ui/constrained_window_constants.h"
@@ skipping 12 matching lines @@
     [self setContentView:contentView];
   }
   return self;
 }
 
 - (id)initWithContentRect:(NSRect)contentRect
                 styleMask:(NSUInteger)windowStyle
                   backing:(NSBackingStoreType)bufferingType
                     defer:(BOOL)deferCreation {
   if ((self = [super initWithContentRect:contentRect
-                               styleMask:windowStyle
+                               styleMask:NSBorderlessWindowMask
                                  backing:bufferingType
                                    defer:NO])) {
     [self setHasShadow:YES];
     [self setBackgroundColor:[NSColor clearColor]];
     [self setOpaque:NO];
     [self setReleasedWhenClosed:NO];
   }
   return self;
 }
 
@@ skipping 26 matching lines @@
                         xRadius:ConstrainedWindowConstants::kBorderRadius
                         yRadius:ConstrainedWindowConstants::kBorderRadius];
   [gfx::SkColorToCalibratedNSColor(
       ConstrainedWindow::GetBackgroundColor()) set];
   [path fill];
 
   [[self window] invalidateShadow];
 }
 
 @end