Prevent a crash when webkit changes the selection.

Partial fix for http://b/issue?id=2081673
This prevents a crash.  However, if the user continues typing
into the textfield, the selection is incorrect.  The real solution
(forthcoming), will make sure that the WebTextView's text gets
updated when webkit updates.

Change-Id: Ic832ec48fd4236c8116c5cbda1467677ad731feb