PPAPI: Disallow blocking callbacks while handling a blocking message

ppapi/api/pp_errors.idl

Index: ppapi/api/pp_errors.idl
diff --git a/ppapi/api/pp_errors.idl b/ppapi/api/pp_errors.idl
index 2d958ecaeeb0d4c9e5bd22b7880e5b38fd0f9b50..85e8506a69d8827e2b330b6f86d05760f4a503eb 100644
--- a/ppapi/api/pp_errors.idl
+++ b/ppapi/api/pp_errors.idl
@@ -140,6 +140,14 @@
    * thread.
    */
   PP_ERROR_WRONG_THREAD   = -52,
+  /**
+   * Returned if you try to use a null completion callback to "block until

[teravest, 2014/09/23 20:23:01]

nit: This doesn't seem consistent with the style of other comments.
nit: "servicing a blocking Javascript is not..." sounds weird.

I'd suggest something more like:
Indicates that a null completion callback was used on a thread handling a
blocking message from JavaScript. Null completion callbacks "block until
complete", but blocking while servicing a blocking Javascript message is not
permitted... and so on.

+   * complete" on a thread that is handling a blocking message from JavaScript.
+   * Using blocking completion callbacks while servicing a blocking JavaScript
+   * is not permitted, because doing so may block the main JavaScript thread
+   * excessively.
+   */
+  PP_ERROR_WOULD_BLOCK_THREAD   = -53,
 
   /**
    * This value indicates that the connection was closed. For TCP sockets, it