[content shell] add scoped allow io to allow for reading files during layout tests

content/shell/shell_message_filter.cc

Index: content/shell/shell_message_filter.cc
diff --git a/content/shell/shell_message_filter.cc b/content/shell/shell_message_filter.cc
index 0aff194224aa5c745b0b75821cfe9a032c58c6d1..cb16871ac15ad8e894be0af8fdca5b0e19969db0 100644
--- a/content/shell/shell_message_filter.cc
+++ b/content/shell/shell_message_filter.cc
@@ -5,6 +5,7 @@
 #include "content/shell/shell_message_filter.h"
 
 #include "base/file_util.h"
+#include "base/threading/thread_restrictions.h"
 #include "content/public/browser/child_process_security_policy.h"
 #include "content/shell/shell_messages.h"
 #include "content/shell/shell_network_delegate.h"
@@ -51,6 +52,7 @@ bool ShellMessageFilter::OnMessageReceived(const IPC::Message& message,
 
 void ShellMessageFilter::OnReadFileToString(const base::FilePath& local_file,
                                             std::string* contents) {
+  base::ThreadRestrictions::ScopedAllowIO allow_io;
   file_util::ReadFileToString(local_file, contents);
 }