Make it possible to remove an Awakable with a specific "context" from a Dispatcher.

mojo/edk/system/message_pipe_dispatcher.cc

Index: mojo/edk/system/message_pipe_dispatcher.cc
diff --git a/mojo/edk/system/message_pipe_dispatcher.cc b/mojo/edk/system/message_pipe_dispatcher.cc
index e57264ae93bc55dc82fb19f84d1c89b1a34675f7..e16548e0b1b99925199ea4fe09a0e13b14d3bb08 100644
--- a/mojo/edk/system/message_pipe_dispatcher.cc
+++ b/mojo/edk/system/message_pipe_dispatcher.cc
@@ -218,6 +218,15 @@ void MessagePipeDispatcher::RemoveAwakableImplNoLock(
   message_pipe_->RemoveAwakable(port_, awakable, signals_state);
 }
 
+void MessagePipeDispatcher::RemoveAwakableWithContextImplNoLock(
+    Awakable* awakable,
+    uint64_t context,
+    HandleSignalsState* signals_state) {
+  mutex().AssertHeld();
+  message_pipe_->RemoveAwakableWithContext(port_, awakable, context,
+                                           signals_state);
+}
+
 void MessagePipeDispatcher::StartSerializeImplNoLock(
     Channel* channel,
     size_t* max_size,