Sync Perl code generator (callback interfaces) with Python changes

The Perl code generator is out of sync with the Python code generator
for callback interfaces, and the generated code doesn't build.
This makes checking code difficult (b/c we have two partial CGs,
not a reference and a new one), and wasn't caught because we don't
run bindings tests on the Python.

This is a minimal change, just fixing the breakage (adds a const)
and rebaselining.

I'll add Python run-bindings-tests in another CL, and fix discrepancies
in the next Python CG CL (mostly consts).

Review URL: https://chromiumcodereview.appspot.com/23074007

git-svn-id: svn://svn.chromium.org/blink/trunk@156468 bbb929c8-8fbe-4397-9dbb-9b2b20218538