Bug 1043518 - Typing a long message or contact-name in SMS app will push the "send" / "add contact" buttons offscreen

This pull request fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1043518 by suppressing the (new, default) min-content-sizing flex item behavior on the flex items that contain the (user-editable) message body & recipient fields, in the SMS compose view.