Fix leading/trailing whitespace form chrome_extension_json output.

Leading and trailing whitespace wasn't included in the output even if marked up
with appropriate ''' delimiters in the .grd file

R=sergeyu@chromium.org

Review URL: https://codereview.chromium.org/242183004

Patch from Peter Lundblad <plundblad@chromium.org>.

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@165 7262f16d-afe8-6277-6482-052fa10e57b1