Support early associated interface binding on ChannelMojo

ipc/ipc_message_pipe_reader.h

Index: ipc/ipc_message_pipe_reader.h
diff --git a/ipc/ipc_message_pipe_reader.h b/ipc/ipc_message_pipe_reader.h
index 5aec4407aaef45842c397d4402e846ba9f4dbd8c..5246478def40933f756fb14e779addb754cbb4fc 100644
--- a/ipc/ipc_message_pipe_reader.h
+++ b/ipc/ipc_message_pipe_reader.h
@@ -13,6 +13,7 @@
 #include "base/atomicops.h"
 #include "base/compiler_specific.h"
 #include "base/macros.h"
+#include "base/process/process_handle.h"
 #include "base/threading/thread_checker.h"
 #include "ipc/ipc.mojom.h"
 #include "ipc/ipc_export.h"
@@ -47,6 +48,7 @@ class IPC_EXPORT MessagePipeReader : public NON_EXPORTED_BASE(mojom::Channel) {
  public:
   class Delegate {
    public:
+    virtual void OnPeerPidReceived() = 0;
     virtual void OnMessageReceived(const Message& message) = 0;
     virtual void OnPipeError() = 0;
     virtual void OnAssociatedInterfaceRequest(
@@ -82,7 +84,6 @@ class IPC_EXPORT MessagePipeReader : public NON_EXPORTED_BASE(mojom::Channel) {
   MessagePipeReader(mojo::MessagePipeHandle pipe,
                     mojom::ChannelAssociatedPtr sender,
                     mojo::AssociatedInterfaceRequest<mojom::Channel> receiver,
-                    base::ProcessId peer_pid,
                     Delegate* delegate);
   ~MessagePipeReader() override;
 
@@ -108,6 +109,7 @@ class IPC_EXPORT MessagePipeReader : public NON_EXPORTED_BASE(mojom::Channel) {
 
  private:
   // mojom::Channel:
+  void SetPeerPid(int32_t peer_pid) override;
   void Receive(mojo::Array<uint8_t> data,
                mojo::Array<mojom::SerializedHandlePtr> handles) override;
   void GetAssociatedInterface(
@@ -116,7 +118,7 @@ class IPC_EXPORT MessagePipeReader : public NON_EXPORTED_BASE(mojom::Channel) {
 
   // |delegate_| is null once the message pipe is closed.
   Delegate* delegate_;
-  base::ProcessId peer_pid_;
+  base::ProcessId peer_pid_ = base::kNullProcessId;
   mojom::ChannelAssociatedPtr sender_;
   mojo::AssociatedBinding<mojom::Channel> binding_;