AX: remove unused AXObject::selectedText.

This CL removes the followings:
 - AXObject::selectedText
 - AXRenderObject::selectedText, which is an override of AXObject::selectedText
 - AXRenderObject::stringForRange and HTMLTextFormControlElement::selectedText,
   which are used only by AXRenderObject::selectedText.

BUG=none
TEST=none; no behavior changes

Review URL: https://codereview.chromium.org/137783006

git-svn-id: svn://svn.chromium.org/blink/trunk@165113 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move dispatchChangeEvent to HTMLFormControlElement

Also devirtualize it along with dispatchInputEvent.

Based on http://trac.webkit.org/changeset/150805 by Andreas Kling.

Review URL: https://codereview.chromium.org/138003005

git-svn-id: svn://svn.chromium.org/blink/trunk@165114 bbb929c8-8fbe-4397-9dbb-9b2b20218538