Images' "Reacting to environment changes" step 5 is triggering spurious downloads

[yoavweiss]

While investigating crbug.com/960633, I realized that the current spec language results in spurious downloads in cases where the image is a no-store or that the cache is disabled.

In Reacting to environment changes, step 5 indicates that only when the chosen URL and image density are the same as before, should the algorithm return without loading a new image. However, for "w" descriptor images with a "vw"-based sizes, every viewport change also results in an image density change, triggering the image's reload.
Those reloads are typically "invisible" as the image is still found in the list of available images, but that breaks when that images cannot be fetched from the cache (as mentioned above, no-store and devtools are cases where I see this happening).

/cc @zcorpan