Found by
https://chromium-review.googlesource.com/c/chromium/src/+/1379025

last_decoded_access_time_ is always 0 (will be removed by the CL).

I also suspect the condition
|prune_frame_time_stamp_ < delay_before_live_decoded_prune_|
is always false in MemoryCache::PruneResources()
because |prune_frame_time_stamp_| is current-time-ish value
(which is a large number of seconds) while
|delay_before_live_decoded_prune_| is 1 sec or so,
and if so we can remove Lines 263-265 and more code.

Or sometimes true, as prune_frame_time_stamp_ is 0 before FramePainter::PaintContents()?

Filing this crbug for tracking and memo.