Fix pending_broker_clients handling

We always remove invitee from pending_invitations_ before adding it to
pending_broker_clients, so the old code actually is buggy and invitees
in pending_broker_clients_ will never be added as a broker client. Fix
it by checking peers_ instead of pending_invitations_.

BUG=b:146518063,b:150661600,b:168250032,chromium:1121709
TEST=manual - Keep running arc.Boot.vm with updated mojo code.

Change-Id: Ib0353944e7d5b9edc04947f8bd2db1442a4ed78d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2411560
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Ken Rockot <rockot@google.com>
Auto-Submit: Lepton Wu <lepton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807076}

mojo/core/node_controller.cc

@@ -993,11 +993,8 @@ void NodeController::OnAcceptBrokerClient(const ports::NodeName& from_node,
   // Feed the broker any pending invitees of our own.
   while (!pending_broker_clients.empty()) {
     const ports::NodeName& invitee_name = pending_broker_clients.front();
-    auto it = pending_invitations_.find(invitee_name);
-    // If for any reason we don't have a pending invitation for the invitee,
-    // there's nothing left to do: we've already swapped the relevant state into
-    // the stack.
-    if (it != pending_invitations_.end()) {
+    auto it = peers_.find(invitee_name);
+    if (it != peers_.end()) {
       broker->AddBrokerClient(invitee_name,
                               it->second->CloneRemoteProcessHandle());
     }