Introduce a delegate to avoid hardcoding "chrome-extension" in net/.

net/proxy/network_delegate_error_observer_unittest.cc

Index: net/proxy/network_delegate_error_observer_unittest.cc
diff --git a/net/proxy/network_delegate_error_observer_unittest.cc b/net/proxy/network_delegate_error_observer_unittest.cc
index 7c8efdff2e7b9786697ef022d0079b6e6d1a3a4f..882fba9b97ac5f11bb59e2720d1e19aef3259f60 100644
--- a/net/proxy/network_delegate_error_observer_unittest.cc
+++ b/net/proxy/network_delegate_error_observer_unittest.cc
@@ -76,6 +76,9 @@ class TestNetworkDelegate : public net::NetworkDelegate {
                                const FilePath& path) const OVERRIDE {
     return true;
   }
+  virtual bool OnCanThrottleRequest(const URLRequest& request) const OVERRIDE {
+    return false;
+  }
 
   bool got_pac_error_;
 };