Loader Rename: Move loader/cache to fetch/

This change moves loader/cache to fetch/, so that several Resouce family clients
no longer depends loader/ and depend fetch/ instead.

This change also moves loader/ResouceLoader and companions to fetch/ because 
ResourceLoader is almost an implementation detail of ResourceFetcher.

BUG=263277
R=abarth,japhet
TEST=none

Review URL: https://chromiumcodereview.appspot.com/22434006

git-svn-id: svn://svn.chromium.org/blink/trunk@155987 bbb929c8-8fbe-4397-9dbb-9b2b20218538