Avoid srcset resource download when higher density resource is in cache.

When resizing the viewport, srcset re-evaluates its resources, and downloads
a new one if needed.

However, since srcset is by definition not related to art-direction,
there's no need to download a new resource is there is a higher density one
that's already in the memory cache.

This CL avoids that unneccessary download.

BUG=425701

Review URL: https://codereview.chromium.org/674923004

git-svn-id: svn://svn.chromium.org/blink/trunk@184335 bbb929c8-8fbe-4397-9dbb-9b2b20218538