Use GUARDED_BY macro in LibassistantServiceHostImpl

This will prevent bugs from forgetting to take the lock.

Bug: b/171748795
Test: chromeos_unittests --gtest_filter="Assistant*"
Change-Id: I6571dba49671c7160b383921541f32261b17bd96
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2601539
Commit-Queue: Jeroen Dhollander <jeroendh@chromium.org>
Auto-Submit: Jeroen Dhollander <jeroendh@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839459}

chromeos/services/assistant/libassistant_service_host_impl.cc

@@ -23,9 +23,8 @@ LibassistantServiceHostImpl::~LibassistantServiceHostImpl() = default;
 
 void LibassistantServiceHostImpl::Launch(
     mojo::PendingReceiver<LibassistantServiceMojom> receiver) {
-  DCHECK_EQ(libassistant_service_, nullptr);
-
   base::AutoLock lock(libassistant_service_lock_);
+  DCHECK(!libassistant_service_);
   libassistant_service_ =
       std::make_unique<chromeos::libassistant::LibassistantService>(
           std::move(receiver), platform_api_, delegate_);

chromeos/services/assistant/libassistant_service_host_impl.h

@@ -51,7 +51,7 @@ class LibassistantServiceHostImpl : public LibassistantServiceHost {
   // methods will be called from the main thread.
   base::Lock libassistant_service_lock_;
   std::unique_ptr<chromeos::libassistant::LibassistantService>
-      libassistant_service_;
+      libassistant_service_ GUARDED_BY(libassistant_service_lock_);
   // Used when SetInitializeCallback() is called before Launch().
   InitializeCallback pending_initialize_callback_;
 };