Support early associated interface binding on ChannelMojo

ipc/ipc_message_pipe_reader.cc

Index: ipc/ipc_message_pipe_reader.cc
diff --git a/ipc/ipc_message_pipe_reader.cc b/ipc/ipc_message_pipe_reader.cc
index 74c72858fcd179b8389583a85619b3eccb7dab19..2bbe91898460ca7efd5c3e6b4cba4e78f774caef 100644
--- a/ipc/ipc_message_pipe_reader.cc
+++ b/ipc/ipc_message_pipe_reader.cc
@@ -55,10 +55,8 @@ MessagePipeReader::MessagePipeReader(
     mojo::MessagePipeHandle pipe,
     mojom::ChannelAssociatedPtr sender,
     mojo::AssociatedInterfaceRequest<mojom::Channel> receiver,
-    base::ProcessId peer_pid,
     MessagePipeReader::Delegate* delegate)
     : delegate_(delegate),
-      peer_pid_(peer_pid),
       sender_(std::move(sender)),
       binding_(this, std::move(receiver)),
       sender_interface_id_(sender_.interface_id()),
@@ -123,9 +121,15 @@ void MessagePipeReader::GetRemoteInterface(
   sender_->GetAssociatedInterface(name, std::move(request));
 }
 
+void MessagePipeReader::SetPeerPid(int32_t peer_pid) {
+  peer_pid_ = peer_pid;
+  delegate_->OnPeerPidReceived();
+}
+
 void MessagePipeReader::Receive(
     mojo::Array<uint8_t> data,
     mojo::Array<mojom::SerializedHandlePtr> handles) {
+  DCHECK_NE(peer_pid_, base::kNullProcessId);
   Message message(
       data.size() == 0 ? "" : reinterpret_cast<const char*>(&data[0]),
       static_cast<uint32_t>(data.size()));