Centralize inner editor readonly handling in TextControlElement.

We handled readonly change on input type=text correctly for the inner
editor, but not for the textarea case. Use common readonly handling for
the inner editor in TextControlElement.

Bug: 931398
Change-Id: Id652fbe5bf75f85ff4bbeba7bb757681c158c30e
Reviewed-on: https://chromium-review.googlesource.com/c/1475760
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633090}
5 files changed