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

net/base/mock_host_resolver.h

Index: net/base/mock_host_resolver.h
diff --git a/net/base/mock_host_resolver.h b/net/base/mock_host_resolver.h
index 033cf147527769bbd3a9cd7eae464a7a7b306037..5542a64cf9da942cce34bfb39936a5f7edd40985 100644
--- a/net/base/mock_host_resolver.h
+++ b/net/base/mock_host_resolver.h
@@ -65,6 +65,14 @@ class MockHostResolverBase : public HostResolver,
     synchronous_mode_ = is_synchronous;
   }
 
+  // Asynchronous requests are automatically resolved by default.
+  // If set_ondemand_mode() is set then Resolve() returns IO_PENDING and
+  // ResolveAllPending() must be explicitly invoked to resolve all requests
+  // that are pending.
+  void set_ondemand_mode(bool is_ondemand) {
+    ondemand_mode_ = is_ondemand;
+  }
+
   // HostResolver methods:
   virtual int Resolve(const RequestInfo& info,
                       AddressList* addresses,
@@ -77,6 +85,15 @@ class MockHostResolverBase : public HostResolver,
   virtual void CancelRequest(RequestHandle req) OVERRIDE;
   virtual HostCache* GetHostCache() OVERRIDE;
 
+  // Resolves all pending requests. It is only valid to invoke this if
+  // set_ondemand_mode was set before. The requests are resolved asynchronously,
+  // after this call returns.
+  void ResolveAllPending();
+
+  // Returns true if there are pending requests that can be resolved by invoking
+  // ResolveAllPending().
+  bool has_pending_requests() const { return !requests_.empty(); }
+
  protected:
   explicit MockHostResolverBase(bool use_caching);
 
@@ -94,6 +111,7 @@ class MockHostResolverBase : public HostResolver,
   void ResolveNow(size_t id);
 
   bool synchronous_mode_;
+  bool ondemand_mode_;
   scoped_refptr<RuleBasedHostResolverProc> rules_;
   scoped_ptr<HostCache> cache_;
   RequestMap requests_;