Fix a crash in FormStructure when parsing a form with zero fields.  Use field_count() instead of directly accessing the size of |fields_| because we NULL-terminate the field vector.
Fix a crash in FormStructure when parsing a form with zero fields.  Use field_count() instead of directly accessing the size of |fields_| because we NULL-terminate the field vector.

BUG=none
TEST=fox.com/24 doesn't crash the browser.
Review URL: http://codereview.chromium.org/657010

git-svn-id: http://src.chromium.org/svn/trunk/src@39778 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 1e81ae07a90d89c81677ff8b0560f80addefa8bd
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