fix: dangling raw_ptr ElectronBrowserContext::extension_system_ (#42785)

The extension system is freed by the DestroyBrowserContextServices()
call in the destructor, so we need to zero out the pointer to avoid
a dangling raw_ptr error.

shell/browser/electron_browser_context.cc

@@ -372,6 +372,10 @@ ElectronBrowserContext::ElectronBrowserContext(
 ElectronBrowserContext::~ElectronBrowserContext() {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   NotifyWillBeDestroyed();
+
+  // the DestroyBrowserContextServices() call below frees this.
+  extension_system_ = nullptr;
+
   // Notify any keyed services of browser context destruction.
   BrowserContextDependencyManager::GetInstance()->DestroyBrowserContextServices(
       this);