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

net/base/filename_util_internal.cc

Index: net/base/filename_util_internal.cc
diff --git a/net/base/filename_util_internal.cc b/net/base/filename_util_internal.cc
index a59de0f50c9174eb4f0cdfbc506428ee695ca57d..0d68bbde377cd65afc6285efc3401172cf9a1448 100644
--- a/net/base/filename_util_internal.cc
+++ b/net/base/filename_util_internal.cc
@@ -171,6 +171,12 @@ void EnsureSafeExtension(const std::string& mime_type,
   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