Add GetMimeTypeFromFile to Mojo's network service.

mojo/services/network/network_service_impl.h

Index: mojo/services/network/network_service_impl.h
diff --git a/mojo/services/network/network_service_impl.h b/mojo/services/network/network_service_impl.h
index 32373610aa12b930cf04ac4634d6a0ad49dee255..5f063ebbfd7e6aba703def0fd2a7b1137b73e497 100644
--- a/mojo/services/network/network_service_impl.h
+++ b/mojo/services/network/network_service_impl.h
@@ -40,6 +40,9 @@ class NetworkServiceImpl : public NetworkService {
   void CreateHttpServer(NetAddressPtr local_address,
                         HttpServerDelegatePtr delegate,
                         const CreateHttpServerCallback& callback) override;
+  void GetMimeTypeFromFile(
+      const mojo::String& file_path,
+      const GetMimeTypeFromFileCallback& callback) override;
 
  private:
   NetworkContext* context_;