Clean up MojoShellConnection in render processes

mojo/shell/runner/host/child_process_host.cc

Index: mojo/shell/runner/host/child_process_host.cc
diff --git a/mojo/shell/runner/host/child_process_host.cc b/mojo/shell/runner/host/child_process_host.cc
index b9a1c1fa7f6e90f09aef556933e5d5fbaa3330aa..d487576345b282e7ebd53d6b115756081fa54b39 100644
--- a/mojo/shell/runner/host/child_process_host.cc
+++ b/mojo/shell/runner/host/child_process_host.cc
@@ -85,15 +85,18 @@ int ChildProcessHost::Join() {
     start_child_process_event_.Wait();
 
   controller_ = mojom::ChildControllerPtr();
-  DCHECK(child_process_.IsValid());
-
-  int rv = -1;
-  LOG_IF(ERROR, !child_process_.WaitForExit(&rv))
-      << "Failed to wait for child process";
+  // This host may be hosting a child process whose lifetime is controlled
+  // elsewhere. In this case we have no known process handle to wait on.
+  if (child_process_.IsValid()) {
+    int rv = -1;
+    LOG_IF(ERROR, !child_process_.WaitForExit(&rv))
+        << "Failed to wait for child process";
 
-  child_process_.Close();
+    child_process_.Close();
+    return rv;
+  }
 
-  return rv;
+  return 0;
 }
 
 void ChildProcessHost::StartApp(
@@ -101,6 +104,9 @@ void ChildProcessHost::StartApp(
     const mojom::ChildController::StartAppCallback& on_app_complete) {
   DCHECK(controller_);
 
+  // In this case the process must have already been launched.
+  start_child_process_event_.Signal();
+
   on_app_complete_ = on_app_complete;
   controller_->StartApp(
       std::move(request),