PPAPI: Make blocking completion callbacks work.

ppapi/thunk/ppb_message_loop_api.h

Index: ppapi/thunk/ppb_message_loop_api.h
diff --git a/ppapi/thunk/ppb_message_loop_api.h b/ppapi/thunk/ppb_message_loop_api.h
index 36f2f1a3a0e00d68294b8ba7c65af293344fd33c..6a73ce1e9ca3f95d0cb08dc1c572f1cfa5356a19 100644
--- a/ppapi/thunk/ppb_message_loop_api.h
+++ b/ppapi/thunk/ppb_message_loop_api.h
@@ -5,6 +5,10 @@
 #ifndef PPAPI_THUNK_PPB_MESSAGE_LOOP_API_H_
 #define PPAPI_THUNK_PPB_MESSAGE_LOOP_API_H_
 
+#include "ppapi/c/pp_bool.h"
+#include "ppapi/c/pp_completion_callback.h"
+#include "ppapi/c/pp_stdint.h"
+
 namespace ppapi {
 namespace thunk {
 
@@ -14,6 +18,10 @@ class PPB_MessageLoop_API {
 
   virtual int32_t AttachToCurrentThread() = 0;
   virtual int32_t Run() = 0;
+  // Note: Most interfaces should use ApiCallbackType, in order to track
+  // callbacks and support things like blocking or optional callbacks. In this
+  // case, the callback is really just a way to pass a function pointer, and
+  // those options don't make sense.
   virtual int32_t PostWork(PP_CompletionCallback callback,
                            int64_t delay_ms) = 0;
   virtual int32_t PostQuit(PP_Bool should_destroy) = 0;