Introduce ERR_NETWORK_CHANGED and allow URLFetcher to automatically retry on that error.

net/base/host_resolver_impl.h

Index: net/base/host_resolver_impl.h
diff --git a/net/base/host_resolver_impl.h b/net/base/host_resolver_impl.h
index 8d28ab37939e021fa277818a9b558fa3fffb1e59..3f7a271d9f60882d4b0175a3cc81541963e182b8 100644
--- a/net/base/host_resolver_impl.h
+++ b/net/base/host_resolver_impl.h
@@ -207,9 +207,9 @@ class NET_EXPORT HostResolverImpl
   // Removes |job| from |jobs_|, only if it exists.
   void RemoveJob(Job* job);
 
-  // Aborts all in progress jobs and notifies their requests.
-  // Might start new jobs.
-  void AbortAllInProgressJobs();
+  // Aborts all in progress jobs with the given |error| and notifies their
+  // requests. Might start new jobs.
+  void AbortAllInProgressJobs(int error);
 
   // Attempts to serve each Job in |jobs_| from the HOSTS file if we have
   // a DnsClient with a valid DnsConfig.