Add error handling and unit test for pdf compositor service

components/printing/service/pdf_compositor_service.h

Index: components/printing/service/pdf_compositor_service.h
diff --git a/components/printing/service/pdf_compositor_service.h b/components/printing/service/pdf_compositor_service.h
index 31672988b5ca84e33d0b896b4b6611b6d20740ae..e88cb4ecad8cca1faf304d4444805aea371d7fc5 100644
--- a/components/printing/service/pdf_compositor_service.h
+++ b/components/printing/service/pdf_compositor_service.h
@@ -34,6 +34,8 @@ class PdfCompositorService : public service_manager::Service {
                        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,