IPC: Fix attachment brokering race condition.

ipc/ipc_channel.h

Index: ipc/ipc_channel.h
diff --git a/ipc/ipc_channel.h b/ipc/ipc_channel.h
index dba2150410cd2cb0edc866b996a00a666d2980e2..fdd0b352964a4a3e554b763c54276dccd0e9bf7e 100644
--- a/ipc/ipc_channel.h
+++ b/ipc/ipc_channel.h
@@ -166,6 +166,9 @@ class IPC_EXPORT Channel : public Endpoint {
   // connect to a pre-existing pipe.  Note, calling Connect()
   // will not block the calling thread and may complete
   // asynchronously.
+  //
+  // The subclass implementation must call WillConnect() at the beginning of its
+  // implementation.
   virtual bool Connect() WARN_UNUSED_RESULT = 0;
 
   // Close this Channel explicitly.  May be called multiple times.
@@ -256,6 +259,16 @@ class IPC_EXPORT Channel : public Endpoint {
     void* buffer_;
     size_t length_;
   };
+
+  // Endpoint overrides.
+  void OnSetAttachmentBrokerEndpoint() override;
+
+  // Subclasses must call this method at the beginning of their implementation
+  // of Connect().
+  void WillConnect();
+
+ private:
+  bool did_start_connect_ = false;
 };
 
 #if defined(OS_POSIX)