Inconsistent image cache key usage

While puzzling over our three image-cache related crash bugs (#8601 (closed), #8559 (closed), and #8618 (closed)), I noticed that the cache key usage was inconsistent in one of the codepaths that caused a crash: We had reversed the first party URI vs the originating URI arguments on one of the calls to GetCacheKey().

I am filing a new ticket because I'm not sure this is the real source of all of the crashes, but it is certainly a bug on its own (possibly resulting in cache isolation flaws and potential third party linkability).

If by some miracle this patch actually does fix all the crashes, we'll just dup them to this bug.