ntp4: show larger favicons in most visited page

extend favicon source to provide larger icons. For now, larger means at most 32x32. Also, the only icon we actually support at this resolution is the default (globe).

BUG=none
TEST=manual

Review URL: http://codereview.chromium.org/7300017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@91517 0039d316-1c4b-4281-b951-d872f2087c98