Access innerTextElement by ID.

ID access is safer than holding a RefPtr because an element accessed by its ID
is always in a DOM tree.

- We need to keep HTMLTextFormControlElement::innerTextElement()
  because some classes call it.
- The ID name is "inner-editor", not "innerTextElement" or something like it.
  "innerTextElement" is confusing with Element::innerText. We're going to rename
  it later.

BUG=none
TEST=Update existing automated tests for internal markup changes.

Review URL: https://codereview.chromium.org/47873002

git-svn-id: svn://svn.chromium.org/blink/trunk@160692 bbb929c8-8fbe-4397-9dbb-9b2b20218538