Change other usages of .size() to .empty() when applicable.

chrome/browser/ui/window_snapshot/window_snapshot_mac.mm

 // Copyright (c) 2011 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.
 
 #include "chrome/browser/ui/window_snapshot/window_snapshot.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/logging.h"
 #include "base/mac/scoped_cftyperef.h"
@@ skipping 17 matching lines @@
 
   scoped_nsobject<NSBitmapImageRep> rep(
       [[NSBitmapImageRep alloc] initWithCGImage:windowSnapshot]);
   NSData* data = [rep representationUsingType:NSPNGFileType properties:nil];
   const unsigned char* buf = static_cast<const unsigned char*>([data bytes]);
   NSUInteger length = [data length];
   if (buf == NULL || length == 0)
     return gfx::Rect();
 
   png_representation->assign(buf, buf + length);
-  DCHECK(png_representation->size() > 0);
+  DCHECK(!png_representation->empty());
 
   return gfx::Rect(static_cast<int>([rep pixelsWide]),
                    static_cast<int>([rep pixelsHigh]));
 }
 
 }  // namespace browser