PPAPI: Disallow blocking callbacks while handling a blocking message

ppapi/shared_impl/ppb_message_loop_shared.h

Index: ppapi/shared_impl/ppb_message_loop_shared.h
diff --git a/ppapi/shared_impl/ppb_message_loop_shared.h b/ppapi/shared_impl/ppb_message_loop_shared.h
index ed7bddb6de0e7675c4b4cb4c81ca645123a9c260..5dd25226f6631e3b16bcdb7b7df67e1398d882d2 100644
--- a/ppapi/shared_impl/ppb_message_loop_shared.h
+++ b/ppapi/shared_impl/ppb_message_loop_shared.h
@@ -50,6 +50,11 @@ class PPAPI_SHARED_EXPORT MessageLoopShared
 
   virtual base::MessageLoopProxy* GetMessageLoopProxy() = 0;
 
+  // Returns whether this MessageLoop is currently handling a blocking message
+  // from JavaScript. This is used to make it illegal to use blocking callbacks
+  // while the thread is handling a blocking message.
+  virtual bool CurrentlyHandlingBlockingMessage() = 0;
+
   DISALLOW_COPY_AND_ASSIGN(MessageLoopShared);
 };