Fix ProcessManagerTest.* flakiness.

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 da355fcc762e17d9d4006842d690f8da7edfabad..7ebfcd968b91c8a309e7299e76880682c60911c1 100644
--- a/extensions/browser/process_manager.cc
+++ b/extensions/browser/process_manager.cc
@@ -248,7 +248,9 @@ ProcessManager::ProcessManager(BrowserContext* context,
   content::DevToolsAgentHost::AddObserver(this);
 }
 
-ProcessManager::~ProcessManager() = default;
+ProcessManager::~ProcessManager() {
+  content::DevToolsAgentHost::RemoveObserver(this);
+}
 
 void ProcessManager::Shutdown() {
   extension_registry_->RemoveObserver(this);