Assert that new renderer processes aren't created during shutdown.

content/browser/browser_main.cc

Index: content/browser/browser_main.cc
diff --git a/content/browser/browser_main.cc b/content/browser/browser_main.cc
index ace957938dd60d0831b59dff35ee7dbcaa217354..1e9fb29040d98a1fc4dbcbce003561395cededbd 100644
--- a/content/browser/browser_main.cc
+++ b/content/browser/browser_main.cc
@@ -252,6 +252,8 @@ void BrowserMainParts::MainMessageLoopStart() {
   PostMainMessageLoopStart();
 }
 
+static bool g_exited_main_message_loop = false;
+
 void BrowserMainParts::RunMainMessageLoopParts() {
   PreMainMessageLoopRun();
 
@@ -262,6 +264,8 @@ void BrowserMainParts::RunMainMessageLoopParts() {
   MainMessageLoopRun();
   TRACE_EVENT_END_ETW("BrowserMain:MESSAGE_LOOP", 0, "");
 
+  g_exited_main_message_loop = true;
+
   PostMainMessageLoopRun();
 }
 
@@ -357,6 +361,10 @@ void BrowserMainParts::PostMainMessageLoopRun() {
 void BrowserMainParts::ToolkitInitialized() {
 }
 
+bool ExitedMainMessageLoop() {
+  return g_exited_main_message_loop;
+}
+
 }  // namespace content
 
 // Main routine for running as the Browser process.