Prevent changes to NavigationEntry's URL for a provisional redirect.

      BUG=77786
TEST=Visit a page that redirects on reload, then stop before it finishes.

Review URL: http://codereview.chromium.org/6793029

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@80639 0039d316-1c4b-4281-b951-d872f2087c98