Change namespace of WebCore::PlainTextRange to WebCore::AXObject::PlainTextRange

This patch changes namespace of Change namespace of WebCore::PlainTextRange to WebCore::AXObject::PlainTextRange for using WebCore::PlainTextRange in editing module, e.g. TextIterator, InputMethodController and WebViewImpl. We don't want to use editing/PlainTextRange in accessibility, because it will have more functions for editing, e.g. PlainTextRange::fromRange(),  PlainTextRange::fromRange(), and so on.

This patch also removes unused member functions, AXObject::selectionStart() and AXObject::selectionEnd().

BUG=n/a
TEST=n/a; no behavior changes
R=dmazzoni@chromium.org, tkent@chromium.org

Review URL: https://codereview.chromium.org/35743007

git-svn-id: svn://svn.chromium.org/blink/trunk@160308 bbb929c8-8fbe-4397-9dbb-9b2b20218538