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

This patch implements srcset and imgsizes attributes support in
HTMLPreloadScanner, behind the Experimental Web Platform feature flag.

Note that the attribute name is temporary and may change as the spec
discussion progresses.

This CL also slightly changes how srcset attribute in <img> is
precessed; now the preload scanner always see only the first srcset=
attribute even if it's empty.

Bug: 813452
Change-Id: If2571a75b6b4d6780e9902be3e8b2ef8d5d887bd
Reviewed-on: https://chromium-review.googlesource.com/923241
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538322}
11 files changed