Make BackgroundContentsService start up BackgroundContents with a delay, as for ExtensionHosts.

extensions/shell/browser/shell_extension_host_delegate.cc

Index: extensions/shell/browser/shell_extension_host_delegate.cc
diff --git a/extensions/shell/browser/shell_extension_host_delegate.cc b/extensions/shell/browser/shell_extension_host_delegate.cc
index d06363e31b83d122f048daefb56b0bc563f40258..51c73341abfaf518f652b99d435fd68729056930 100644
--- a/extensions/shell/browser/shell_extension_host_delegate.cc
+++ b/extensions/shell/browser/shell_extension_host_delegate.cc
@@ -4,7 +4,9 @@
 
 #include "extensions/shell/browser/shell_extension_host_delegate.h"
 
+#include "base/lazy_instance.h"
 #include "base/logging.h"
+#include "extensions/browser/serial_extension_host_queue.h"
 #include "extensions/shell/browser/media_capture_util.h"
 #include "extensions/shell/browser/shell_extension_web_contents_observer.h"
 
@@ -61,4 +63,11 @@ bool ShellExtensionHostDelegate::CheckMediaAccessPermission(
   return true;
 }
 
+static base::LazyInstance<SerialExtensionHostQueue> g_queue =
+    LAZY_INSTANCE_INITIALIZER;
+
+ExtensionHostQueue* ShellExtensionHostDelegate::GetExtensionHostQueue() const {
+  return g_queue.Pointer();
+}
+
 }  // namespace extensions