Make sure channel-associated interface pointers are always safe to call.

ipc/ipc_sync_message_filter.cc

Index: ipc/ipc_sync_message_filter.cc
diff --git a/ipc/ipc_sync_message_filter.cc b/ipc/ipc_sync_message_filter.cc
index ffccff3d50889271ac0b4c0926b10f62eb9e4ea8..b9737fd29379b6dcfc640e4a764fef1684b16efd 100644
--- a/ipc/ipc_sync_message_filter.cc
+++ b/ipc/ipc_sync_message_filter.cc
@@ -276,8 +276,13 @@ void SyncMessageFilter::GetGenericRemoteAssociatedInterface(
     mojo::ScopedInterfaceEndpointHandle handle) {
   base::AutoLock auto_lock(lock_);
   DCHECK(io_task_runner_ && io_task_runner_->BelongsToCurrentThread());
-  if (!channel_)
+  if (!channel_) {
+    // Attach the associated interface to a disconnected pipe, so that the
+    // associated interface pointer can be used to make calls (which are
+    // dropped).
+    mojo::GetIsolatedInterface(std::move(handle));
     return;
+  }
 
   Channel::AssociatedInterfaceSupport* support =
       channel_->GetAssociatedInterfaceSupport();