Non-activated default buttons prevent implicit form submission.

Follow the spec and two other implementations (Gecko + Presto), and do
not allow implicit form submission if the default (submit) button is
not in an activated state (read: disabled.)

Notice that this switches to behavior different from what Trident / IE
provides.

R=eseidel,tkent
BUG=37402

Review URL: https://codereview.chromium.org/60573007

git-svn-id: svn://svn.chromium.org/blink/trunk@162095 bbb929c8-8fbe-4397-9dbb-9b2b20218538