Apply dangling markup restrictions to `<link>`.

`preconnect`, `dns-prefetch`, and `prefetch` were all inadvertantly
bypassing the danging markup mitigations we landed for "actual" resource
requests. This patch resolves that oversight.

TBR=mkwst@chromium.org

(cherry picked from commit 682b16cc3fd2316946670f25f38a9abba6827fe4)

Bug: 680970, 695474, 749852
Change-Id: Ic2a262d062a92830b1869b3fb3183280156f3c0a
Reviewed-on: https://chromium-review.googlesource.com/571785
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: Yoav Weiss <yoav@yoav.ws>
Cr-Original-Commit-Position: refs/heads/master@{#493728}
Reviewed-on: https://chromium-review.googlesource.com/620587
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/branch-heads/3163@{#671}
Cr-Branched-From: ff259bab28b35d242e10186cd63af7ed404fae0d-refs/heads/master@{#488528}
5 files changed