Simplify how variable_map is initialized. Suggestion by thakis@chromium.org.

BUG=https://code.google.com/p/grit-i18n/issues/detail?id=23

Review URL: https://codereview.chromium.org/122253002

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@142 7262f16d-afe8-6277-6482-052fa10e57b1