Add a non-blocking "OneShotEvent" class

chrome/browser/extensions/test_extension_system.cc

Index: chrome/browser/extensions/test_extension_system.cc
diff --git a/chrome/browser/extensions/test_extension_system.cc b/chrome/browser/extensions/test_extension_system.cc
index fcb55d68db439451b90de41f5f47b4c192af03e9..959f0b67c3234e880a413d3613a454966fb25d51 100644
--- a/chrome/browser/extensions/test_extension_system.cc
+++ b/chrome/browser/extensions/test_extension_system.cc
@@ -100,7 +100,8 @@ ExtensionService* TestExtensionSystem::CreateExtensionService(
                                                 ExtensionPrefs::Get(profile_),
                                                 blacklist_.get(),
                                                 autoupdate_enabled,
-                                                true));
+                                                true,
+                                                &ready_));
   extension_service_->ClearProvidersForTesting();
   return extension_service_.get();
 }
@@ -176,6 +177,10 @@ Blacklist* TestExtensionSystem::blacklist() {
   return blacklist_.get();
 }
 
+const OneShotEvent& TestExtensionSystem::ready() const {
+  return ready_;
+}
+
 // static
 ProfileKeyedService* TestExtensionSystem::Build(
     content::BrowserContext* profile) {