Disallow UI/IO thread blocking on any other thread.

net/disk_cache/in_flight_io.cc

Index: net/disk_cache/in_flight_io.cc
===================================================================
--- net/disk_cache/in_flight_io.cc	(revision 133458)
+++ net/disk_cache/in_flight_io.cc	(working copy)
@@ -7,6 +7,7 @@
 #include "base/bind.h"
 #include "base/location.h"
 #include "base/logging.h"
+#include "base/threading/thread_restrictions.h"
 
 namespace disk_cache {
 
@@ -82,7 +83,11 @@
 
 // Runs on the primary thread.
 void InFlightIO::InvokeCallback(BackgroundIO* operation, bool cancel_task) {
-  operation->io_completed()->Wait();
+  {
+    // http://crbug.com/74623
+    base::ThreadRestrictions::ScopedAllowWait allow_wait;
+    operation->io_completed()->Wait();
+  }
   running_ = true;
 
   if (cancel_task)