Add error handling and unit test for pdf compositor service

components/printing/service/pdf_compositor_service.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_PRINTING_SERVICE_PDF_COMPOSITOR_SERVICE_H_
 #define COMPONENTS_PRINTING_SERVICE_PDF_COMPOSITOR_SERVICE_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 15 matching lines @@
   // Factory function for use as an embedded service.
   static std::unique_ptr<service_manager::Service> Create(
       const std::string& creator);
 
   // service_manager::Service:
   void OnStart() override;
   void OnBindInterface(const service_manager::BindSourceInfo& source_info,
                        const std::string& interface_name,
                        mojo::ScopedMessagePipeHandle interface_pipe) override;
 
+  virtual void PrepareToStart();
+
  private:
   // The creator of this service.
   // Currently contains the service creator's user agent string if given,
   // otherwise just use string "Chromium".
   const std::string creator_;
 
   std::unique_ptr<discardable_memory::ClientDiscardableSharedMemoryManager>
       discardable_shared_memory_manager_;
   std::unique_ptr<service_manager::ServiceContextRefFactory> ref_factory_;
   service_manager::BinderRegistry registry_;
   base::WeakPtrFactory<PdfCompositorService> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(PdfCompositorService);
 };
 
 }  // namespace printing
 
 #endif  // COMPONENTS_PRINTING_SERVICE_PDF_COMPOSITOR_SERVICE_H_