Pass pref_task_runner through HttpServerPropertiesManager's cxtor so that tests could inject a Test…

net/http/http_server_properties_manager.h

Index: net/http/http_server_properties_manager.h
diff --git a/net/http/http_server_properties_manager.h b/net/http/http_server_properties_manager.h
index e1c47ff913daa3d72a5e848cc3e17f88dc215d68..15fb05335aa7ea3c4dbb49d2798c79a966833797 100644
--- a/net/http/http_server_properties_manager.h
+++ b/net/http/http_server_properties_manager.h
@@ -84,6 +84,7 @@ class NET_EXPORT HttpServerPropertiesManager : public HttpServerProperties {
   // Must be constructed on the Pref thread.

[brettw, 2016/12/13 23:26:13]

Can you add a description here of the two task runners, why you need two, and
what each one will be used for. I actually don't understand myself so I'll
re-review after that. Thanks!

[Zhongyi Shi, 2016/12/14 19:47:24]

Sorry about the confusion. I have updated the CL description, ptal. Hope that
helps reviewing.

[brettw, 2016/12/14 23:16:54]

Thanks for the description update. But I think we need to update this comment as
well. Somebody calling this function isn't going to find this CL description to
see why there are two task runners and what each one should be.

[Zhongyi Shi, 2016/12/14 23:36:58]

Oh, sorry that I miss the comments in the headers files. Updated now. 

   HttpServerPropertiesManager(
       PrefDelegate* pref_delegate,
+      scoped_refptr<base::SingleThreadTaskRunner> pref_task_runner,
       scoped_refptr<base::SingleThreadTaskRunner> network_task_runner);
   ~HttpServerPropertiesManager() override;