Make submitting a POST with Ctrl/Shift to work

This patch changes a way of loading submitting a POST form with modifiers, e.g. Ctrl/Shift, as same as a form with target attribute instead of OpenURL() of content layer, which doesn't support form data.

BUG=76665
R=mkwst@chromium.org

Review URL: https://codereview.chromium.org/19597003

git-svn-id: svn://svn.chromium.org/blink/trunk@154818 bbb929c8-8fbe-4397-9dbb-9b2b20218538