Add hook into pepper to query if running on the main thread.

webkit/glue/plugins/pepper_plugin_module.cc

Index: webkit/glue/plugins/pepper_plugin_module.cc
diff --git a/webkit/glue/plugins/pepper_plugin_module.cc b/webkit/glue/plugins/pepper_plugin_module.cc
index 3b4486889da8ba513651ac06d23ea5c137347a5c..6af71dc8e8f7739fa748e85a3280f84d1f6ceb2a 100644
--- a/webkit/glue/plugins/pepper_plugin_module.cc
+++ b/webkit/glue/plugins/pepper_plugin_module.cc
@@ -120,13 +120,18 @@ void CallOnMainThread(int delay_in_msec,
       delay_in_msec);
 }
 
+bool IsMainThread() {
+  return GetMainThreadMessageLoop()->BelongsToCurrentThread();
+}
+
 const PPB_Core core_interface = {
   &AddRefResource,
   &ReleaseResource,
   &MemAlloc,
   &MemFree,
   &GetTime,
-  &CallOnMainThread
+  &CallOnMainThread,
+  &IsMainThread
 };
 
 // PPB_Testing -----------------------------------------------------------------