merge_pr_6
Form submission reentrancy protection for 'submit' and 'invalid' event
Differential Revision: https://phabricator.services.mozilla.com/D53696
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1551758
gecko-commit: 1f77dd98df61bb0b6c5ce53befbb98635fe4eed8
gecko-integration-branch: autoland
gecko-reviewers: smaug