Have content/ create and destroy its own threads.

chrome/browser/browser_shutdown.cc

Index: chrome/browser/browser_shutdown.cc
diff --git a/chrome/browser/browser_shutdown.cc b/chrome/browser/browser_shutdown.cc
index 64a3a06e914555cea4d7adcaa88255dae1227e2c..a3bc10ed0e042b2683650920515c7b0ea2f707ed 100644
--- a/chrome/browser/browser_shutdown.cc
+++ b/chrome/browser/browser_shutdown.cc
@@ -113,7 +113,7 @@ FilePath GetShutdownMsPath() {
   return shutdown_ms_file.AppendASCII(kShutdownMsFile);
 }
 
-void Shutdown() {
+bool ShutdownPreThreadsStop() {
 #if defined(OS_CHROMEOS)
   chromeos::BootTimesLoader::Get()->AddLogoutTimeMarker(
       "BrowserShutdownStarted", false);
@@ -161,6 +161,10 @@ void Shutdown() {
   RLZTracker::CleanupRlz();
 #endif
 
+  return restart_last_session;
+}
+
+void ShutdownPostThreadsStop(bool restart_last_session) {
   // The jank'o'meter requires that the browser process has been destroyed
   // before calling UninstallJankometer().
   delete g_browser_process;