Partially reland r131930. Only assert on system context leaks this time.

Original CL:
==========
Assert we do not leak requests for the system and safe browsing contexts.

It's possible this will catch some leaks when deployed live. We should be able to analyze the crash dumps and fix the culprits.
==========

This time, I'll only assert we do not leak requests for the system
request context, since I've confirmed that we are leaking requests for
the safe browsing context.

BUG=90971
TEST=None
TBR=eroman


Review URL: http://codereview.chromium.org/10096017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@132531 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/io_thread.cc

@@ -465,6 +465,8 @@ void IOThread::CleanUp() {
   net::ShutdownNSSHttpIO();
 #endif  // defined(USE_NSS)
 
+  globals_->system_request_context->AssertNoURLRequests();
+
   system_url_request_context_getter_ = NULL;
 
   // Release objects that the net::URLRequestContext could have been pointing