Restore KeyedServiceFactory diagnostics for context use-after-destroy.

extensions/browser/process_manager.cc

Index: extensions/browser/process_manager.cc
diff --git a/extensions/browser/process_manager.cc b/extensions/browser/process_manager.cc
index dc3b5e71266cc0a0b0c4ed56d36d244f6ed572b3..1f18b4eff54563a8fe3bdde771e7afac1aef42c0 100644
--- a/extensions/browser/process_manager.cc
+++ b/extensions/browser/process_manager.cc
@@ -267,11 +267,14 @@ ProcessManager::ProcessManager(BrowserContext* context,
   OnKeepaliveImpulseCheck();
 }
 
-ProcessManager::~ProcessManager() {
+ProcessManager::~ProcessManager() = default;
+
+void ProcessManager::Shutdown() {
   extension_registry_->RemoveObserver(this);
   CloseBackgroundHosts();
   DCHECK(background_hosts_.empty());
   content::DevToolsAgentHost::RemoveObserver(this);
+  site_instance_ = nullptr;
 }
 
 void ProcessManager::RegisterRenderFrameHost(