Remove some dead code in ThumbnailGenerator.

chrome/browser/ui/cocoa/tabpose_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/tabpose_window.h"
 
 #import <QuartzCore/QuartzCore.h>
 
 #include <algorithm>
 
@@ skipping 168 matching lines @@
   // keep it simple and request full thumbnails for everything.
   // TODO(thakis): Request smaller thumbnails for users with many tabs.
   gfx::Size page_size(size_);  // Logical size the renderer renders at.
   gfx::Size pixel_size(size_);  // Physical pixel size the image is rendered at.
 
   generator->MonitorRenderer(rwh_, true);
 
   // Will send an IPC to the renderer on the IO thread.
   generator->AskForSnapshot(
       rwh_,
-      /*prefer_backing_store=*/false,
       base::Bind(&ThumbnailLoader::DidReceiveBitmap,
                  weak_factory_.GetWeakPtr()),
       page_size,
       pixel_size);
 }
 
 }  // namespace tabpose
 
 @implementation ThumbnailLayer
 
@@ skipping 1466 matching lines @@
   tile.set_tab_contents(contents);
   ThumbnailLayer* thumbLayer = [allThumbnailLayers_ objectAtIndex:index];
   [thumbLayer setTabContents:contents];
 }
 
 - (void)tabStripModelDeleted {
   [self close];
 }
 
 @end