Handle brokered attachment race condition

ipc/attachment_broker.cc

Index: ipc/attachment_broker.cc
diff --git a/ipc/attachment_broker.cc b/ipc/attachment_broker.cc
index 45fe1204681395aefc7218e8475dcdfbd784b2e4..df3e724632852d6299efb5ace06c096d1303327d 100644
--- a/ipc/attachment_broker.cc
+++ b/ipc/attachment_broker.cc
@@ -58,6 +58,16 @@ void AttachmentBroker::AddObserver(
     info.runner = runner;
     info.unique_id = ++last_unique_id_;
     observers_.push_back(info);
+
+    // Give the observer a chance to handle attachments that arrived while the
+    // observer was handling the message that caused it to register, but our
+    // mutex was not yet locked.
+    for (const auto& attachment : attachments_) {
+      info.runner->PostTask(
+          FROM_HERE,
+          base::Bind(&AttachmentBroker::NotifyObserver, base::Unretained(this),
+                     info.unique_id, attachment->GetIdentifier()));
+    }
   }
 }