Referrer policy is not initialized in HTMLLinkElement, giving it the
value kReferrerPolicyAlways, meaning that the proper referrer policy is
ignored for link elements.

This causes rel="prefetch" linked resources to always have the full
referrer, contradicting the spec: https://w3c.github.io/resource-hints/:
"the user agent must [...] Apply relevant Referrer policy restrictions
for resource hint link's, as defined by [REFERRER-POLICY]."

WPT test written by [email protected].

Bug: 758597
Change-Id: Iedb3c6197798413d76e41bcccb06a19407daeb62
Reviewed-on: https://chromium-review.googlesource.com/635623
Commit-Queue: Benoit L <[email protected]>
Reviewed-by: Yoav Weiss <[email protected]>
Reviewed-by: Jochen Eisinger <[email protected]>
Cr-Commit-Position: refs/heads/master@{#497781}