Add DCHECK for error_string so future NULL pointers can be caught
Add DCHECK for error_string so future NULL pointers can be caught

Review URL: http://codereview.chromium.org/523138

git-svn-id: http://src.chromium.org/svn/trunk/src@35786 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: cb6bee7dce9a4c178063ecc0556f4c5dd8b17929
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser