Fix potential crashes in NetworkHandler code

chrome/browser/chromeos/chrome_browser_main_chromeos.cc

Index: chrome/browser/chromeos/chrome_browser_main_chromeos.cc
diff --git a/chrome/browser/chromeos/chrome_browser_main_chromeos.cc b/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
index 71dc2fbdeedf51d9f66ec2414547b9bb07da2917..ce294e29904adc2821a9c2798fd36533e9e5fe57 100644
--- a/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
+++ b/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
@@ -25,6 +25,7 @@
 #include "base/sys_info.h"
 #include "base/task_runner_util.h"
 #include "chrome/browser/browser_process.h"
+#include "chrome/browser/browser_process_impl.h"
 #include "chrome/browser/browser_process_platform_part_chromeos.h"
 #include "chrome/browser/chrome_notification_types.h"
 #include "chrome/browser/chromeos/accessibility/accessibility_manager.h"
@@ -76,6 +77,7 @@
 #include "chrome/browser/chromeos/system/input_device_settings.h"
 #include "chrome/browser/chromeos/upgrade_detector_chromeos.h"
 #include "chrome/browser/defaults.h"
+#include "chrome/browser/io_thread.h"
 #include "chrome/browser/lifetime/application_lifetime.h"
 #include "chrome/browser/net/chrome_network_delegate.h"
 #include "chrome/browser/profiles/profile.h"
@@ -823,6 +825,10 @@ void ChromeBrowserMainPartsChromeos::PostMainMessageLoopRun() {
 }
 
 void ChromeBrowserMainPartsChromeos::PostDestroyThreads() {
+  // Destroy the ProxyConfigServiceImpl owned by IOThread before

[sky, 2016/01/05 23:17:11]

This just documents the code. Document why this order is important.

[stevenjb, 2016/01/06 00:43:04]

Clarified.

+  // shutting down NetworkHandler.
+  browser_process()->io_thread()->ShutdownPrefProxyConfigTracker();

[sky, 2016/01/05 23:17:11]

Won't this mean you're destroying the PrefProxyConfigTracker on a different
thread than it is currently destroyed? Is it setup to handle that?

[stevenjb, 2016/01/06 00:43:04]

No, it is created in IOThread() which is (somewhat unintuitively) created on the
UI thread. I added a few DCHECK_CURRENTLY_ON(UI) calls in io_thread.cc to
document that.

[sky, 2016/01/06 16:30:37]

I'm nervous and not familiar enough with this code to know if this is correct.
Can you get someone that knows about the threading of PrefProxyConfigTracker to
review this?

+
   // Destroy DBus services immediately after threads are stopped.
   dbus_services_.reset();