Fix incorrect template per-world separation in a custom binding.

The same bug in the generated code was fixed in
https://codereview.chromium.org/19218002/.

BUG=

Review URL: https://chromiumcodereview.appspot.com/22568002

git-svn-id: svn://svn.chromium.org/blink/trunk@155688 bbb929c8-8fbe-4397-9dbb-9b2b20218538