Fix form resubmissions happening silently.

BUG=263508
TEST=http/tests/history/back-to-post.html

Review URL: https://chromiumcodereview.appspot.com/20369002

git-svn-id: svn://svn.chromium.org/blink/trunk@155005 bbb929c8-8fbe-4397-9dbb-9b2b20218538