Add a non-blocking "OneShotEvent" class

chrome/browser/extensions/extension_system.cc

Index: chrome/browser/extensions/extension_system.cc
diff --git a/chrome/browser/extensions/extension_system.cc b/chrome/browser/extensions/extension_system.cc
index 0a141447866847d3153f84ec583dde0392dae13f..cc9bad93a1e6016130da3794a538ac3399c40b54 100644
--- a/chrome/browser/extensions/extension_system.cc
+++ b/chrome/browser/extensions/extension_system.cc
@@ -436,6 +436,14 @@ Blacklist* ExtensionSystemImpl::blacklist() {
   return shared_->blacklist();
 }
 
+const OneShotEvent& ExtensionSystemImpl::ready() const {
+  return *shared_->ready();
+}
+
+void ExtensionSystemImpl::ExtensionServiceReady() {
+  shared_->ready()->Signal();
+}
+
 void ExtensionSystemImpl::RegisterExtensionWithRequestContexts(
     const Extension* extension) {
   base::Time install_time;