Implement a poor man's PostAfterStartupTask() function.

content/browser/browser_main_loop.cc

Index: content/browser/browser_main_loop.cc
diff --git a/content/browser/browser_main_loop.cc b/content/browser/browser_main_loop.cc
index 2409bc78f8a26ef634559cd92a218278da491bca..bb5bb1f1d37f02d48c0fc72b09de61bbc2673c92 100644
--- a/content/browser/browser_main_loop.cc
+++ b/content/browser/browser_main_loop.cc
@@ -21,6 +21,7 @@
 #include "base/threading/thread_restrictions.h"
 #include "base/timer/hi_res_timer_manager.h"
 #include "base/trace_event/trace_event.h"
+#include "content/browser/after_startup_task_poster_impl.h"
 #include "content/browser/browser_thread_impl.h"
 #include "content/browser/device_sensors/device_inertial_sensor_service.h"
 #include "content/browser/download/save_file_manager.h"
@@ -586,6 +587,9 @@ void BrowserMainLoop::MainMessageLoopStart() {
 }
 
 int BrowserMainLoop::PreCreateThreads() {
+  SetAfterStartupTaskPoster(
+      GetContentClient()->browser()->GetAfterStartupTaskPoster());
+
   if (parts_) {
     TRACE_EVENT0("startup",
         "BrowserMainLoop::CreateThreads:PreCreateThreads");