Add error handling and unit test for pdf compositor service

components/printing/service/service_main.cc

+// 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.
+
+#include "base/test/test_discardable_memory_allocator.h"
+#include "components/printing/service/pdf_compositor_service.h"
+#include "services/service_manager/public/c/main.h"
+#include "services/service_manager/public/cpp/service_runner.h"
+
+// In order to test PdfCompositorService, this class overrides and
+// uses a test discardable memory allocator.
+class PdfCompositorTestService : public printing::PdfCompositorService {
+ public:
+  explicit PdfCompositorTestService(const std::string& creator)
+      : PdfCompositorService(creator) {}
+
+  // PdfCompositorService:
+  void PrepareToStart() override;

[Lei Zhang, 2017/06/02 21:54:09]

Need a dtor that overrides.

[Wei Li, 2017/07/07 21:23:57]

Done.

+};
+
+void PdfCompositorTestService::PrepareToStart() {
+  base::TestDiscardableMemoryAllocator mem_allocator;
+  base::DiscardableMemoryAllocator::SetInstance(&mem_allocator);
+}
+
+MojoResult ServiceMain(MojoHandle service_request_handle) {
+  service_manager::ServiceRunner runner(
+      new PdfCompositorTestService("pdf_compositor_service_unittest"));
+  return runner.Run(service_request_handle);
+}