Lazily initialize MessageLoop for faster thread startup

content/public/test/test_browser_thread.h

Index: content/public/test/test_browser_thread.h
diff --git a/content/public/test/test_browser_thread.h b/content/public/test/test_browser_thread.h
index 2428bbb66ba729f3a61c33056d3d0a581ff5957b..d3a256474e36c40814ab9037a8def9c10a39ba2f 100644
--- a/content/public/test/test_browser_thread.h
+++ b/content/public/test/test_browser_thread.h
@@ -36,6 +36,10 @@ class TestBrowserThread {
   // Starts the thread with a generic message loop.
   bool Start();
 
+  // Starts the thread with a generic message loop and waits for the
+  // thread to run.
+  bool StartAndWaitForTesting();
+
   // Starts the thread with an IOThread message loop.
   bool StartIOThread();