aura: Make the Linux version of IconLoader not require GTK+.

chrome/browser/icon_manager_linux.cc

 // 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/icon_manager.h"
 
 #include "base/mime_util.h"
 #include "base/threading/thread_restrictions.h"
 
 IconGroupID IconManager::GetGroupIDFromFilepath(const FilePath& filepath) {
-#if defined(USE_AURA)
-  // TODO(davemoore) Implement this for aura.
-  return std::string();
-#else
   // It turns out the call to mime_util::GetFileMimeType below does IO, but
   // callers of GetGroupIDFromFilepath assume it does not do IO (the Windows
   // and Mac implementations do not). We should fix this by either not doing IO
   // in this method, or reworking callers to avoid calling it on the UI thread.
   // See crbug.com/72740.
   base::ThreadRestrictions::ScopedAllowIO allow_io;
 
   return mime_util::GetFileMimeType(filepath);
-#endif
 }