Reland: 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 55767a73ccc42be11fb5aca1574bbc6f027d6d32..33a0d410f8e889b492f2746fddd9b1b02711c876 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(