Total guess fix for the bug....

chrome/browser/cocoa/download_shelf_controller.mm

 // Copyright (c) 2009 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/cocoa/download_shelf_controller.h"
 
 #include "app/l10n_util.h"
 #include "app/resource_bundle.h"
 #include "base/mac_util.h"
 #include "base/sys_string_conversions.h"
@@ skipping 214 matching lines @@
     currentX += frame.size.width + kDownloadItemPadding;
     skipFirst = NO;
   }
 }
 
 - (void)layoutItems {
   [self layoutItems:NO];
 }
 
 - (void)addDownloadItem:(BaseDownloadItemModel*)model {
+  DCHECK([NSThread isMainThread]);
   // Insert new item at the left.
   scoped_nsobject<DownloadItemController> controller(
       [[DownloadItemController alloc] initWithModel:model shelf:self]);
 
   // Adding at index 0 in NSMutableArrays is O(1).
   [downloadItemControllers_ insertObject:controller.get() atIndex:0];
 
   [itemContainerView_ addSubview:[controller view]];
 
   // The controller is in charge of removing itself as an observer in its
@@ skipping 49 matching lines @@
     if (isTransferDone &&
         [itemController download]->safety_state() != DownloadItem::DANGEROUS) {
       [self remove:itemController];
     } else {
       ++i;
     }
   }
 }
 
 @end