Convert resources to take an instance key instead of an Instance*.

ppapi/cpp/module.h

Index: ppapi/cpp/module.h
diff --git a/ppapi/cpp/module.h b/ppapi/cpp/module.h
index a9af8e0d024b61e0b452914eb3074ab121f602e7..871128c79b93f0e05aa28740529c22b4ec2e0a20 100644
--- a/ppapi/cpp/module.h
+++ b/ppapi/cpp/module.h
@@ -86,7 +86,10 @@ class Module {
   const void* GetBrowserInterface(const char* interface_name);
 
   /// InstanceForPPInstance() returns the object associated with this
-  /// <code>PP_Instance</code>, or NULL if one is not found.
+  /// <code>PP_Instance</code>, or NULL if one is not found. This should only
+  /// be called from the main thread! This instance object may be destroyed at
+  /// any time on the main thread, so using it on other threads may cause a
+  /// crash.
   ///
   /// @param[in] instance This <code>PP_Instance</code>.
   ///