Fix checkValidity for form containing menulist select

checkvalidity for form returns false when we select the option from menulist. This happens because setNeedsValidityCheck() is called after dispatchInputAndChangeEventForMenuList() is called. Moved the setNeedsValidityCheck() before dispatchInputAndChangeEventForMenuList().

BUG=360444
TEST=fast/forms/checkValidity-select-form.html

Review URL: https://codereview.chromium.org/246883004

git-svn-id: svn://svn.chromium.org/blink/trunk@172386 bbb929c8-8fbe-4397-9dbb-9b2b20218538