Fix potential crashes in NetworkHandler code

chrome/browser/io_thread.cc

Index: chrome/browser/io_thread.cc
diff --git a/chrome/browser/io_thread.cc b/chrome/browser/io_thread.cc
index cc7872f5efc962fa7e2d6533fe823dad312db444..e9aca8b10ae5a72d0507754e9dbf440e80a54a78 100644
--- a/chrome/browser/io_thread.cc
+++ b/chrome/browser/io_thread.cc
@@ -556,8 +556,7 @@ IOThread::~IOThread() {
   // This isn't needed for production code, but in tests, IOThread may
   // be multiply constructed.
   BrowserThread::SetDelegate(BrowserThread::IO, NULL);
-
-  pref_proxy_config_tracker_->DetachFromPrefService();
+  ShutdownPrefProxyConfigTracker();
   DCHECK(!globals_);
 }
 
@@ -893,6 +892,13 @@ void IOThread::Init() {
                                      weak_factory_.GetWeakPtr()));
 }
 
+void IOThread::ShutdownPrefProxyConfigTracker() {
+  if (!pref_proxy_config_tracker_)
+    return;
+  pref_proxy_config_tracker_->DetachFromPrefService();
+  pref_proxy_config_tracker_.reset();
+}
+
 void IOThread::CleanUp() {
   base::debug::LeakTracker<SafeBrowsingURLRequestContext>::CheckForLeaks();