Fix Crash when shutting down after starting Assistant

This could lead to the following chain of events:
    - (Browser thread) Starts Assistant, and calls mojom method
      |ServiceController::Start()|.
    - (Browser thread) Stops Assistant, and calls mojom method
      |ServiceController::Stop()|.
    - (Mojom thread) ServiceController::Start()
           - Creates |AssistantManager|
           - Publishes it through the singleton |LibassistantV1Api|.
           - Informs browser thread through mojom method
             |StateObserver::OnStateChanged(started)|.
    - Here we get an interwoven sequence of events between the mojom
      thread and the browser thread:
      - (Mojom thread) ServiceController::Stop()
             - Informs browser thread through mojom method
               |StateObserver::OnStateChanged(stopped)|.
             - (Here we are 'interrupted' by the browser thread)
      - (Browser thread) StateObserver::OnStateChanged(started) calls
        AssistantManagerServiceImpl::PostInitAssistant() which
        accesses the |LibassistantV1Api| which is not null yet.
      - (Mojom thread) now destroys |LibassistantV1Api| and
        |AssistantManager|.
      - (Browser thread) dereferences the (now deleted)
        |AssistantManager| object and crashes.

Bug: 1162136
Test: Compiled and deployed
Change-Id: I86b30c76c544a926e46e7817540f17f0910da192
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612402
Commit-Queue: Jeroen Dhollander <jeroendh@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840883}

chromeos/services/assistant/proxy/service_controller_proxy.cc

@@ -212,7 +212,6 @@ void ServiceControllerProxy::OnStateChanged(ServiceState new_state) {
 
   switch (new_state) {
     case ServiceState::kStarted:
-      DCHECK_EQ(state_, State::kStarting);
       FinishCreatingAssistant();
       break;
     case ServiceState::kRunning:
@@ -237,6 +236,16 @@ ServiceControllerProxy::assistant_manager_internal() {
 }
 
 void ServiceControllerProxy::FinishCreatingAssistant() {
+  if (state_ == State::kStopped) {
+    // We can come here if the system went into shutdown while the mojom
+    // service was busy starting Libassistant.
+    // This means the |AssistantManager| could be destroyed at any second,
+    // so we simply clean up and bail out.
+    on_start_done_callback_.reset();
+    pending_display_connection_ = nullptr;
+    return;
+  }
+
   DCHECK(on_start_done_callback_.has_value());
   DCHECK_NE(pending_display_connection_, nullptr);