Tweaks to improve memory consumption by TopSites. The biggest culprit

seems to repeatedly querying history during this perf test. So, the
biggest gain is made by scaling this back. Here are other tweaks I've
made:

. decrease the page size/cache size of the two DBs. Now that favicons
  doesn't store thumbnails the db doesn't need to be so big.
. Set a cap to the max number of tmp thumnbails maintains.
. Force jpg data vector to be only as big as it needs to be.
. Tweak TopSitesCache so it doesn't end up duplicating so many
  GURLs. This is particularly helpful for long redirect lists.

BUG=61487
TEST=none

Review URL: http://codereview.chromium.org/4106014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@64837 0039d316-1c4b-4281-b951-d872f2087c98