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The API returns 503 errors if we try to download photos too
quickly.  This change adds cheesy rate-limiting so we don't
request more than 10 photos per second (which appears to be
the undocumented limit) and also makes us retry when we get
a 503.

I'm also making GDataContactsService report success even
when we got 404 errors for some photos, as I see this happen
with my personal account. :-/

BUG=128805
TEST=none


Review URL: https://chromiumcodereview.appspot.com/10823182

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@150344 0039d316-1c4b-4281-b951-d872f2087c98