[content] Fix ConnectionFilter registration

Fixes the ordering between ContentClient::OnServiceManagerConnected()
and ServiceManagerConnection::Start() in ChildThreadImpl and
UtilityThreadIMpl, ensuring embedders can register their own ConnectionFilters
prior to incoming interface requests being dispatched on the IO thread.

Bug: 894369
Change-Id: Ibaa9290904f1c5004b53111aec687385e4e80a9d
Reviewed-on: https://chromium-review.googlesource.com/c/1279172
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599393}

content/child/child_thread_impl.cc

@@ -743,9 +743,13 @@ void ChildThreadImpl::OnAssociatedInterfaceRequest(
 
 void ChildThreadImpl::StartServiceManagerConnection() {
   DCHECK(service_manager_connection_);
-  service_manager_connection_->Start();
   GetContentClient()->OnServiceManagerConnected(
       service_manager_connection_.get());
+
+  // NOTE: You must register any ConnectionFilter instances on
+  // |service_manager_connection_| *before* this call to |Start()|, otherwise
+  // incoming interface requests may race with the registration.
+  service_manager_connection_->Start();
 }
 
 bool ChildThreadImpl::OnControlMessageReceived(const IPC::Message& msg) {

content/utility/utility_thread_impl.cc

@@ -151,8 +151,12 @@ void UtilityThreadImpl::Init() {
   service_factory_.reset(new UtilityServiceFactory);
 
   if (connection) {
-    connection->Start();
     GetContentClient()->OnServiceManagerConnected(connection);
+
+    // NOTE: You must register any ConnectionFilter instances on |connection|
+    // *before* this call to |Start()|, otherwise incoming interface requests
+    // may race with the registration.
+    connection->Start();
   }
 }