src: add lock to inspector MainThreadHandle dtor

Otherwise, the `CHECK` is reported to be a race condition
by automated tooling. It’s not easy to tell from looking at
the source code whether that is actually the case or not,
but adding this lock should be a safe way to resolve it.

PR-URL: #26010
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: addaleax

src/inspector/main_thread_interface.h

@@ -45,6 +45,7 @@ class MainThreadHandle : public std::enable_shared_from_this<MainThreadHandle> {
                             : main_thread_(main_thread) {
   }
   ~MainThreadHandle() {
+    Mutex::ScopedLock scoped_lock(block_lock_);
     CHECK_NULL(main_thread_);  // main_thread_ should have called Reset
   }
   std::unique_ptr<InspectorSession> Connect(