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 6f7bdb8c2c9583f8d42eb6293766703c7d85f1cf..4c1bb5dae001f3e202af74558700185382a02c44 100644
--- a/components/printing/service/pdf_compositor_service.h
+++ b/components/printing/service/pdf_compositor_service.h
@@ -33,6 +33,8 @@ 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,