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.

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-Commit-Position: refs/heads/master@{#493728}
5 files changed