PPAPI: Disallow blocking callbacks while handling a blocking message

ppapi/proxy/message_handler.cc

Index: ppapi/proxy/message_handler.cc
diff --git a/ppapi/proxy/message_handler.cc b/ppapi/proxy/message_handler.cc
index e095dd76509a84465b2953d362ea02f4db51ce18..a0f5cccb18393e58f0971c97652505abc8b07a9a 100644
--- a/ppapi/proxy/message_handler.cc
+++ b/ppapi/proxy/message_handler.cc
@@ -39,8 +39,12 @@ void HandleBlockingMessageWrapper(HandleBlockingMessageFunc function,
   if (!dispatcher)
     return;
   PP_Var result = PP_MakeUndefined();
+  MessageLoopResource::GetCurrent()->
+      set_currently_handling_blocking_message(true);
   CallWhileUnlocked(
       function, instance, user_data, &message_data.get(), &result);
+  MessageLoopResource::GetCurrent()->
+      set_currently_handling_blocking_message(false);
   PpapiMsg_PPPMessageHandler_HandleBlockingMessage::WriteReplyParams(
       reply_msg.get(),
       SerializedVarReturnValue::Convert(dispatcher, result),
@@ -64,10 +68,14 @@ void HandleBlockingMessageWrapper_0_1(HandleBlockingMessageFunc_0_1 function,
   PluginDispatcher* dispatcher = PluginDispatcher::GetForInstance(instance);
   if (!dispatcher)
     return;
+  MessageLoopResource::GetCurrent()->
+      set_currently_handling_blocking_message(true);
   PP_Var return_value = CallWhileUnlocked(function,
                                           instance,
                                           user_data,
                                           message_data.get());
+  MessageLoopResource::GetCurrent()->
+      set_currently_handling_blocking_message(false);
   PpapiMsg_PPPMessageHandler_HandleBlockingMessage::WriteReplyParams(
       reply_msg.get(),
       SerializedVarReturnValue::Convert(dispatcher, return_value),