Remove a TextIteratorBehavior enum which is not used in Blink.

This patch removes an enum value, TextIteratorEmitsObjectReplacementCharacters,
from TextIteratorBehavior. It was originally introduced for WebKit's GTK+ port,
and apparently was not used in Chromium at all.

BUG=none
R=hayato@chromium.org,eseidel@chromium.org

Review URL: https://codereview.chromium.org/77703002

git-svn-id: svn://svn.chromium.org/blink/trunk@162369 bbb929c8-8fbe-4397-9dbb-9b2b20218538