Revert of PPAPI: Fix GetBrowserInterface race conditions

ppapi/proxy/plugin_dispatcher.cc

Index: ppapi/proxy/plugin_dispatcher.cc
diff --git a/ppapi/proxy/plugin_dispatcher.cc b/ppapi/proxy/plugin_dispatcher.cc
index ad3e7a066429e8197c10501680bfb414f68b5564..f6448d380c2a3b23aa4ef7f3fff37e82df81b998 100644
--- a/ppapi/proxy/plugin_dispatcher.cc
+++ b/ppapi/proxy/plugin_dispatcher.cc
@@ -108,8 +108,13 @@
 
 // static
 const void* PluginDispatcher::GetBrowserInterface(const char* interface_name) {
-  // CAUTION: This function is called directly from the plugin, but we *don't*
-  // lock the ProxyLock to avoid excessive locking from C++ wrappers.
+  if (!interface_name) {
+    DLOG(WARNING) << "|interface_name| is null. Did you forget to add "
+        "the |interface_name()| template function to the interface's C++ "
+        "wrapper?";
+    return NULL;
+  }
+
   return InterfaceList::GetInstance()->GetInterfaceForPPB(interface_name);
 }