ipc: Change the timing of the construction of the attachment broker.

ipc/attachment_broker.cc

Index: ipc/attachment_broker.cc
diff --git a/ipc/attachment_broker.cc b/ipc/attachment_broker.cc
index 362fc1f52235b138553dd6d1d33effdd8d7a79a5..7e04bcd645de09dd4a4a32e3cd8f6198c567926d 100644
--- a/ipc/attachment_broker.cc
+++ b/ipc/attachment_broker.cc
@@ -14,10 +14,7 @@ namespace IPC {
 
 // static
 void AttachmentBroker::SetGlobal(AttachmentBroker* broker) {
-  // TODO(erikchen): There should be a CHECK here to make sure that
-  // |g_attachment_broker| is nullptr. Right now, this causes problems with
-  // --single_process and similar testing conditions. I'm temporarily removing
-  // this CHECK. http://crbug.com/534539.
+  CHECK(!g_attachment_broker);
   g_attachment_broker = broker;
 }