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.
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.
Review URL: http://codereview.chromium.org/20028

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@9105 0039d316-1c4b-4281-b951-d872f2087c98

adobe/chromium,adobe/chromium,yitian134/chromium,gavinp/chromium,adobe/chromium,adobe/chromium,yitian134/chromium,ropik/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,yitian134/chromium,gavinp/chromium,ropik/chromium,gavinp/chromium,gavinp/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,ropik/chromium,ropik/chromium,gavinp/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,adobe/chromium,gavinp/chromium,gavinp/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,adobe/chromium,ropik/chromium,yitian134/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,adobe/chromium,Crystalnix/house-of-life-chromium,ropik/chromium,yitian134/chromium,ropik/chromium,ropik/chromium,Crystalnix/house-of-life-chromium,yitian134/chromium,gavinp/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,yitian134/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,gavinp/chromium