Support generic synchronization against an IPC::Channel

ipc/ipc.mojom

Index: ipc/ipc.mojom
diff --git a/ipc/ipc.mojom b/ipc/ipc.mojom
index 0a4fcfa84e60f05925955604007d4830376a7f94..8318818d10c4dc5cdb5db62374706cb1d628dceb 100644
--- a/ipc/ipc.mojom
+++ b/ipc/ipc.mojom
@@ -4,6 +4,8 @@
 
 module IPC.mojom;
 
+import "mojo/common/event.mojom";
+
 // NOTE: This MUST match the value of MSG_ROUTING_NONE in src/ipc/ipc_message.h.
 const int32 kRoutingIdNone = -2;
 
@@ -34,6 +36,10 @@ interface Channel {
 
   // Requests a Channel-associated interface.
   GetAssociatedInterface(string name, associated GenericInterface& request);
+
+  // Signals |event| immediately upon receipt. Useful for synchronizing
+  // arbitrary clients against the Channel's message FIFO.
+  SignalEvent(mojo.common.mojom.Event event);
 };
 
 // A strictly nominal interface used to identify Channel bootstrap requests.