PPAPI: Make blocking completion callbacks work.

ppapi/shared_impl/ppapi_globals.h

Index: ppapi/shared_impl/ppapi_globals.h
diff --git a/ppapi/shared_impl/ppapi_globals.h b/ppapi/shared_impl/ppapi_globals.h
index d83e9c6ca7d7ddd905954bb029a2decdd4e45491..f3f8441e730c6d85a3692872fedd3e78bd277713 100644
--- a/ppapi/shared_impl/ppapi_globals.h
+++ b/ppapi/shared_impl/ppapi_globals.h
@@ -17,6 +17,7 @@
 
 namespace base {
 class Lock;
+class MessageLoopProxy;
 }
 
 namespace ppapi {
@@ -95,6 +96,8 @@ class PPAPI_SHARED_EXPORT PpapiGlobals {
   // failure.
   virtual PP_Module GetModuleForInstance(PP_Instance instance) = 0;
 
+  base::MessageLoopProxy* GetMainThreadMessageLoop();
+
   virtual bool IsHostGlobals() const;
   virtual bool IsPluginGlobals() const;