Revert "Revert "Don't crash by form submission in a document with null encoding.""

This reverts r159463. It turns out it was not the cause of the test failure.

TBR=tkent@chromium.org

Review URL: https://codereview.chromium.org/27065002

git-svn-id: svn://svn.chromium.org/blink/trunk@159474 bbb929c8-8fbe-4397-9dbb-9b2b20218538