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 6f56d0e447d48c969ae8835d398eb7d3923ecb31..7c4099847ac096f0d8133cf813dc089d513d910b 100644
--- a/chrome/browser/extensions/extension_system.cc
+++ b/chrome/browser/extensions/extension_system.cc
@@ -141,7 +141,8 @@ void ExtensionSystemImpl::Shared::Init(bool extensions_enabled) {
       ExtensionPrefs::Get(profile_),
       blacklist_.get(),
       autoupdate_enabled,
-      extensions_enabled));
+      extensions_enabled,
+      &ready_));
 
   // These services must be registered before the ExtensionService tries to
   // load any extensions.
@@ -409,6 +410,10 @@ Blacklist* ExtensionSystemImpl::blacklist() {
   return shared_->blacklist();
 }
 
+const OneShotEvent& ExtensionSystemImpl::ready() const {
+  return shared_->ready();
+}
+
 void ExtensionSystemImpl::RegisterExtensionWithRequestContexts(
     const Extension* extension) {
   base::Time install_time;