Directly set block_third_party_cookies_ when observing a preference change to avoid a NOTREACHED.
Directly set block_third_party_cookies_ when observing a preference change to avoid a NOTREACHED.

BUG=45644
TEST=No crash when changing "Block third-party cookies" preference with an incognito window open.

Review URL: http://codereview.chromium.org/2423003

git-svn-id: http://src.chromium.org/svn/trunk/src@49148 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 6f3a1c745233424c687a9b29d60ebbf16bf315ef
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