Reland (3rd try): Lazily initialize MessageLoop for faster thread startup

chrome/browser/io_thread.cc

Index: chrome/browser/io_thread.cc
diff --git a/chrome/browser/io_thread.cc b/chrome/browser/io_thread.cc
index 46d0e3e85bb2b3a2f26c65eb7ad8bf5862ce3640..907ffbdf7d9e032de3c9f8403da2aacee1d90049 100644
--- a/chrome/browser/io_thread.cc
+++ b/chrome/browser/io_thread.cc
@@ -608,11 +608,6 @@ net::URLRequestContextGetter* IOThread::system_url_request_context_getter() {
 }
 
 void IOThread::Init() {
-  // Prefer to use InitAsync unless you need initialization to block
-  // the UI thread
-}
-
-void IOThread::InitAsync() {
   // TODO(erikchen): Remove ScopedTracker below once http://crbug.com/466432
   // is fixed.
   tracked_objects::ScopedTracker tracking_profile1(