[Mac] Handle shift-delete when omnibox popup is up.

[Mac] Handle shift-delete when omnibox popup is up.

When the user hits shift-delete (that's forward delete),
attempt to delete the item highlighted from the list.  This only seems
to work for history items.

http://crbug.com/20281
TEST=No idea.  I've managed to delete an item, but most items seem
immune to deletion, this code is calling the right place, but the
match itself checks !deletable.  Sometimes I have luck going to a
site, then typing part of the URL and using shift-del on it.

[Scott Hess - ex-Googler, 2009-08-28 20:28:31]

A little bird has informed me that this one is shift-delete on Windows, which
means it's shift-deleteForward for us.  I've verified that that's the key for
Windows Chrome and Mac FF.

[Scott Hess - ex-Googler, 2009-08-28 20:38:39]

On 2009/08/28 20:28:31, shess wrote:
> A little bird has informed me that this one is shift-delete on Windows, which
> means it's shift-deleteForward for us.  I've verified that that's the key for
> Windows Chrome and Mac FF.

Now using forward delete.

[rohitrao (ping after 24h), 2009-08-28 21:59:14]

LGTM

My laptop doesn't have a delete key =(