Restore base::ThreadRestrictions::ScopedAllowIO to see if it fixes linux bot failures.

net/base/filename_util_internal.cc

 // Copyright 2014 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 "net/base/filename_util.h"
 
 #include "base/file_util.h"
 #include "base/files/file_path.h"
 #include "base/path_service.h"
 #include "base/strings/string_util.h"
@@ skipping 153 matching lines @@
                          bool ignore_extension,
                          base::FilePath* file_name) {
   // See if our file name already contains an extension.
   base::FilePath::StringType extension = file_name->Extension();
   if (!extension.empty())
     extension.erase(extension.begin());  // Erase preceding '.'.
 
   if ((ignore_extension || extension.empty()) && !mime_type.empty()) {
     base::FilePath::StringType preferred_mime_extension;
     std::vector<base::FilePath::StringType> all_mime_extensions;
+    // The GetPreferredExtensionForMimeType call will end up going to disk.  Do
+    // this on another thread to avoid slowing the IO thread.
+    // http://crbug.com/61827
+    // TODO(asanka): Remove this ScopedAllowIO once all callers have switched
+    // over to IO safe threads.
+    base::ThreadRestrictions::ScopedAllowIO allow_io;
     net::GetPreferredExtensionForMimeType(mime_type, &preferred_mime_extension);
     net::GetExtensionsForMimeType(mime_type, &all_mime_extensions);
     // If the existing extension is in the list of valid extensions for the
     // given type, use it. This avoids doing things like pointlessly renaming
     // "foo.jpg" to "foo.jpeg".
     if (std::find(all_mime_extensions.begin(),
                   all_mime_extensions.end(),
                   extension) != all_mime_extensions.end()) {
       // leave |extension| alone
     } else if (!preferred_mime_extension.empty()) {
@@ skipping 125 matching lines @@
   base::FilePath generated_name(
       base::SysWideToNativeMB(base::UTF16ToWide(file_name)));
 #endif
 
   DCHECK(!generated_name.empty());
 
   return generated_name;
 }
 
 }  // namespace net