Fix a crash where we would try to add a DocumentMarker with a...

Fix a crash where we would try to add a 
DocumentMarker with the same start- and endOffset, 
which AddMarker in WebKit will ignore. As a result,
our marker vector is empty and we try to access 
the element with index -1.

Also added a check to avoid a NOTREACHED in debug
build, which can happen under normal circumstances 
(when you navigate away from a page when a search 
is in progress).

BUG=http://crbug.com/7250
TEST=We don't have reproduction steps for the
crash but if we stop seeing this on our crash
server we'll know that it is fixed.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=9105

[sky, 2009-02-03 21:23:59]

OK