Add srcset/imgsizes support for <link rel=preload> in LinkLoader

This patch teaches LinkLoader to recognize srcset / imgsizes attributes
when preloading images.

This feature is behind the Experimental Web Platform feature flag. Also
note that the attribute name is temporary and may change as the spec
discussion progresses.

Bug: 813452
Change-Id: I82770edd7a99ec328c3748f192f98cf9df0af274
Reviewed-on: https://chromium-review.googlesource.com/927930
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538365}
8 files changed