[Extensions] Handle some funny cases in script injection

extensions/renderer/script_injection_manager.h

Index: extensions/renderer/script_injection_manager.h
diff --git a/extensions/renderer/script_injection_manager.h b/extensions/renderer/script_injection_manager.h
index a0416b389e133755afcfbab794066ed423866a33..88f552071f323b15bf13e579457d5faf9b79d394 100644
--- a/extensions/renderer/script_injection_manager.h
+++ b/extensions/renderer/script_injection_manager.h
@@ -99,6 +99,9 @@ class ScriptInjectionManager : public UserScriptSetManager::Observer {
   // RunLocation of the frame corresponds to the last location that has ran.
   FrameStatusMap frame_statuses_;
 
+  // The frames currently being injected into, so long as that frame is valid.
+  std::set<content::RenderFrame*> active_injection_frames_;
+
   // The collection of RFOHelpers.
   ScopedVector<RFOHelper> rfo_helpers_;