Fix crash when updating a navigation entry without a site_instance.
Fix crash when updating a navigation entry without a site_instance.

R=creis
BUG=http://crbug.com/12423

Review URL: http://codereview.chromium.org/115695

git-svn-id: http://src.chromium.org/svn/trunk/src@16729 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 7f8b9ebfdc2fc47b004943e790769f2bf2ac0e7e
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser