Add imageSrcset and imageSizes IDL attributes to HTMLLinkElement

This feature is behind the Experimental Web Platform feature flag.

Spec PR: https://github.com/whatwg/html/pull/4048

Bug: 813452
Change-Id: I84c58035789b64a311637cebc4672d16661980c2
Reviewed-on: https://chromium-review.googlesource.com/c/1350558
Reviewed-by: Dominic Farolino <domfarolino@gmail.com>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610990}
4 files changed