Post a task to the IO thread to add a ResourceContext in ResourceDispatcherHostImpl.

content/browser/loader/resource_dispatcher_host_impl.cc

Index: content/browser/loader/resource_dispatcher_host_impl.cc
diff --git a/content/browser/loader/resource_dispatcher_host_impl.cc b/content/browser/loader/resource_dispatcher_host_impl.cc
index 022f410c82590f05aa1ddc8bf3f01a1c6a174bfc..0b8009013f9973f5c0d8db1424d70c0f08700828 100644
--- a/content/browser/loader/resource_dispatcher_host_impl.cc
+++ b/content/browser/loader/resource_dispatcher_host_impl.cc
@@ -543,6 +543,7 @@ void ResourceDispatcherHostImpl::SetAllowCrossOriginAuthPrompt(bool value) {
 }
 
 void ResourceDispatcherHostImpl::AddResourceContext(ResourceContext* context) {
+  DCHECK_CURRENTLY_ON(BrowserThread::IO);

[mmenke, 2016/01/21 16:33:34]

May want to add a DCHECK to RemoveResourceContext as well, just for
documentation purposes (And out of paranoia)

[Anand Mistry (off Chromium), 2016/01/21 19:41:52]

The reason I didn't is because of the comment about tests destroying the
ResourceContext on the UI thread.

   active_resource_contexts_.insert(context);
 }