Rename innerTextElement/Value used in <input> and <textarea> to innerEditor.

'innerText' is super-confusing because we have Element::innerText and
Element::setInnerText.

BUG=
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/340273003

git-svn-id: svn://svn.chromium.org/blink/trunk@176483 bbb929c8-8fbe-4397-9dbb-9b2b20218538