linux_aura: run IconLoader::ReadIcon on the UI thread.

chrome/browser/icon_loader_mac.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.
 
 #include "chrome/browser/icon_loader.h"
 
 #import <AppKit/AppKit.h>
 
 #include "base/bind.h"
 #include "base/files/file_path.h"
 #include "base/message_loop/message_loop.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/threading/thread.h"
 #include "ui/gfx/image/image_skia.h"
 #include "ui/gfx/image/image_skia_util_mac.h"
 
 // static
 IconGroupID IconLoader::ReadGroupIDFromFilepath(
     const base::FilePath& filepath) {
   return filepath.Extension();
 }
 
+// static
 bool IconLoader::IsIconMutableFromFilepath(const base::FilePath&) {
   return false;
 }
 
+// static
+content::BrowserThread::ID IconLoader::ReadIconThreadID() {
+  return content::BrowserThread::FILE;
+}
+
 void IconLoader::ReadIcon() {
   NSString* group = base::SysUTF8ToNSString(group_);
   NSWorkspace* workspace = [NSWorkspace sharedWorkspace];
   NSImage* icon = [workspace iconForFileType:group];
 
   if (icon_size_ == ALL) {
     // The NSImage already has all sizes.
     image_.reset(new gfx::Image([icon retain]));
   } else {
     NSSize size = NSZeroSize;
@@ skipping 10 matching lines @@
     gfx::ImageSkia image_skia(gfx::ImageSkiaFromResizedNSImage(icon, size));
     if (!image_skia.isNull()) {
       image_skia.MakeThreadSafe();
       image_.reset(new gfx::Image(image_skia));
     }
   }
 
   target_message_loop_->PostTask(FROM_HERE,
       base::Bind(&IconLoader::NotifyDelegate, this));
 }