Don't extend delete selection from zero.

The previous fix ensured that trying to delete something actually did delete something but ended up also ensuring deletion even when trying to delete nothing.  Now we don't attempt to extend the range when the requested range started at zero.

BUG=355995

Review URL: https://codereview.chromium.org/381883004

git-svn-id: svn://svn.chromium.org/blink/trunk@178679 bbb929c8-8fbe-4397-9dbb-9b2b20218538