Add error handling and unit test for pdf compositor service

components/printing/service/pdf_compositor_service.cc

Index: components/printing/service/pdf_compositor_service.cc
diff --git a/components/printing/service/pdf_compositor_service.cc b/components/printing/service/pdf_compositor_service.cc
index d00f3ed76bd6584f26f1e7f72351950d3aa39a5f..d7f4b2a815443e0b2104b48e3b4f0bb7fde61a1c 100644
--- a/components/printing/service/pdf_compositor_service.cc
+++ b/components/printing/service/pdf_compositor_service.cc
@@ -43,7 +43,7 @@ std::unique_ptr<service_manager::Service> PdfCompositorService::Create(
   return base::MakeUnique<printing::PdfCompositorService>(creator);
 }
 
-void PdfCompositorService::OnStart() {
+void PdfCompositorService::PrepareToStart() {
   // Set up discardable memory manager.
   discardable_memory::mojom::DiscardableSharedMemoryManagerPtr manager_ptr;
   context()->connector()->BindInterface(content::mojom::kBrowserServiceName,
@@ -54,6 +54,10 @@ void PdfCompositorService::OnStart() {
   DCHECK(discardable_shared_memory_manager_);
   base::DiscardableMemoryAllocator::SetInstance(
       discardable_shared_memory_manager_.get());
+}
+
+void PdfCompositorService::OnStart() {
+  PrepareToStart();
 
   ref_factory_ = base::MakeUnique<service_manager::ServiceContextRefFactory>(
       base::Bind(&service_manager::ServiceContext::RequestQuit,