Make srcset on link rel=preload work with SignedHTTPExchange flag too

<link rel=preload> will play an important role in signed-exchange
prefetching. This patch makes srcset/imgsizes on <link rel=preload> work
with the SignedHTTPExchange flag too (in addition to the Experimental
Web Platform features flag), so that people can easily try out it.

Bug: 803774,813452
Change-Id: I976d007c73b9cb5459d35e0e178f8d8d90484709
Reviewed-on: https://chromium-review.googlesource.com/961627
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@{#543029}
3 files changed