Move common methods to WebFormControlElement interface

There are some common methods in Input element, TextArea element and
Select element. By moving these methods to WebFormControlElement
interface, it will simplify Chromium side code.

R=isherman@chromium.org, tkent@chromium.org
BUG=351255

Review URL: https://codereview.chromium.org/194923003

git-svn-id: svn://svn.chromium.org/blink/trunk@168923 bbb929c8-8fbe-4397-9dbb-9b2b20218538