Introduce incognito preference settings.

net/proxy/multi_threaded_proxy_resolver.cc

Index: net/proxy/multi_threaded_proxy_resolver.cc
diff --git a/net/proxy/multi_threaded_proxy_resolver.cc b/net/proxy/multi_threaded_proxy_resolver.cc
index ad1228c90b9a55a427022f97faf04230cd779e1d..c5922725e63daffaf0c76058242b29047a4fa353 100644
--- a/net/proxy/multi_threaded_proxy_resolver.cc
+++ b/net/proxy/multi_threaded_proxy_resolver.cc
@@ -8,6 +8,7 @@
 #include "base/string_util.h"
 #include "base/stringprintf.h"
 #include "base/threading/thread.h"
+#include "base/threading/thread_restrictions.h"
 #include "net/base/net_errors.h"
 #include "net/base/net_log.h"
 #include "net/proxy/proxy_info.h"
@@ -337,7 +338,12 @@ void MultiThreadedProxyResolver::Executor::Destroy() {
   resolver_->Shutdown();
 
   // Join the worker thread.
-  thread_.reset();
+  {
+    // Allow IO because joining can block for a long time and is required
+    // by thread destructor.
+    base::ThreadRestrictions::ScopedAllowIO allow_io;
+    thread_.reset();
+  }
 
   // Cancel any outstanding job.
   if (outstanding_job_) {