Fix thumbnail retain code to retain more than the first url.

The re-used statement was not being reset between loops, so only data
for the first url was retained.

BUG=283878,272519
TEST=Open bookmark bar.  Create some bookmarks with recognizable favicons.  With only NNTP open, clear entire history.  Quit.  Restart, favicons should still be there.

Review URL: https://codereview.chromium.org/32653004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230171 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230171