Fix another off-by-one error in computeEllipsis

In the START case, we can iterate back before the start of the
string.  A similar fix was previously applied to the MIDDLE
case of computeEllipsis:

commit 0e3c5e82
Author: Raph Levien <raph@google.com>
Date:   Thu Dec 4 13:26:07 2014 -0800

    Fix off-by-one error in computeEllipsis

Fixes JIRA: NIGHTLIES-1280.

Change-Id: I081eaec9bce5a8989c8fc1b0d440848b40a66158
(cherry picked from commit d1689f3b)