https://codereview.chromium.org/1117663002 contained a copy-paste
error; this is a follow-up fixing that so that error messages now
contain the correct filename when using the --extra option to js2c.py.

R=yangguo@chromium.org, jochen@chromium.org

Review URL: https://codereview.chromium.org/1120593002

Cr-Commit-Position: refs/heads/master@{#28158}