Support generic synchronization against an IPC::Channel

ipc/ipc_channel.h

Index: ipc/ipc_channel.h
diff --git a/ipc/ipc_channel.h b/ipc/ipc_channel.h
index 2861d0f224814f4b0ca107087e7f789be9363cd0..6243148e4b567fbe2cd81dc6ef9e8acb36f6aaa3 100644
--- a/ipc/ipc_channel.h
+++ b/ipc/ipc_channel.h
@@ -21,6 +21,7 @@
 #include "ipc/ipc_channel_handle.h"
 #include "ipc/ipc_endpoint.h"
 #include "ipc/ipc_message.h"
+#include "mojo/common/event.mojom.h"
 #include "mojo/public/cpp/bindings/associated_group.h"
 #include "mojo/public/cpp/bindings/associated_interface_ptr.h"
 #include "mojo/public/cpp/bindings/associated_interface_request.h"
@@ -266,6 +267,11 @@ class IPC_EXPORT Channel : public Endpoint {
   // deleted once the contents of the Message have been sent.
   bool Send(Message* message) override = 0;
 
+  // Sends an Event pipe to the remote endpoint which will Signal() it
+  // immediately upon receipt. This can be used to synchronize against the
+  // Channel's message FIFO.
+  virtual void SendEventToSignal(mojo::common::mojom::EventPtr event);
+
   // NaCl in Non-SFI mode runs on Linux directly, and the following functions
   // compiled on Linux are also needed. Please see also comments in
   // components/nacl_nonsfi.gyp for more details.