Improve <textarea> validation performance.

HTMLTextAreaElement::value() is a costly operation, and we can avoid it
if a <textarea> has neither 'required' attribute nor 'maxlength' attribute.

This CL improves PerformanceTests/DOM/textarea-dom.html and
textarea-edit.html by a few percent.

BUG=

Review URL: https://codereview.chromium.org/543403003

git-svn-id: svn://svn.chromium.org/blink/trunk@181785 bbb929c8-8fbe-4397-9dbb-9b2b20218538