Use PostTaskAndReplyWithResult in plugin_service_impl.cc.

content/test/fake_plugin_service.h

Index: content/test/fake_plugin_service.h
diff --git a/content/test/fake_plugin_service.h b/content/test/fake_plugin_service.h
index 082b646f6d1f85f9f5ae7dd80c0264a90914b55e..db23a83ed079a31fc757813f500266c02541bc49 100644
--- a/content/test/fake_plugin_service.h
+++ b/content/test/fake_plugin_service.h
@@ -40,7 +40,7 @@ class FakePluginService : public PluginService {
                            WebPluginInfo* info) override;
   base::string16 GetPluginDisplayNameByPath(
       const base::FilePath& path) override;
-  void GetPlugins(const GetPluginsCallback& callback) override;
+  void GetPlugins(GetPluginsCallback callback) override;
   PepperPluginInfo* GetRegisteredPpapiPluginInfo(
       const base::FilePath& plugin_path) override;
   void SetFilter(PluginServiceFilter* filter) override;