Make grit resource maps generate maps for strings.

This allows mapping from "IDS_FOO" to IDS_FOO to
get generated string identifiers.

Patch by sigbjorn@opera.com.  Code review at
https://codereview.chromium.org/22739002/ .

R=tony

Review URL: https://codereview.chromium.org/23090002

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@133 7262f16d-afe8-6277-6482-052fa10e57b1