Making sure that base::MessagePumpForUI from different modules are isolated from each other.

base/message_pump_win.h

Index: base/message_pump_win.h
diff --git a/base/message_pump_win.h b/base/message_pump_win.h
index f5a00f3015ecc4a2e61f57e07a44311eb4fdb8ec..cf6bff99485f00b3c8e940bc32e4a46cd131f5b7 100644
--- a/base/message_pump_win.h
+++ b/base/message_pump_win.h
@@ -154,6 +154,9 @@ class BASE_EXPORT MessagePumpForUI : public MessagePumpWin {
   bool ProcessMessageHelper(const MSG& msg);
   bool ProcessPumpReplacementMessage();
 
+  // Instance of the module containing the window procedure.
+  HMODULE instance_;
+
   // A hidden message-only window.
   HWND message_hwnd_;
 };