Enable PreloadImageSrcSet feature by default

This enables imagesrcset and imagesizes attributes on <link>.

Intent to Ship:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/07CbItWfA4M/CG1qF8IoEAAJ

Bug: 813452
Change-Id: I9396fe35729069ef56b461aa99cf85e9b1e51302
Reviewed-on: https://chromium-review.googlesource.com/c/1414513
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623170}
3 files changed