[Approach 2] Accumulate tasks in a proxy task runner until thread starts.

content/public/test/test_browser_thread.cc

Index: content/public/test/test_browser_thread.cc
diff --git a/content/public/test/test_browser_thread.cc b/content/public/test/test_browser_thread.cc
index ac67480fe5cd36b6cd515d866656e2687983a9f1..aa689f91613cdaf68d38f6f99f5e10b9ed980e8f 100644
--- a/content/public/test/test_browser_thread.cc
+++ b/content/public/test/test_browser_thread.cc
@@ -58,6 +58,10 @@ bool TestBrowserThread::Start() {
   return impl_->Start();
 }
 
+bool TestBrowserThread::StartAndWait() {
+  return impl_->StartAndWait();
+}
+
 bool TestBrowserThread::StartIOThread() {
   base::Thread::Options options;
   options.message_loop_type = base::MessageLoop::TYPE_IO;