Only (and always) put URLs in NSLinkAttributeName.
When the NSLinkAttributeName attribute is used, AppKit automatically turns it into a draggable link. If anything but a URL is used, that link then becomes a bomb to explode drag targets. This change ensures that only URLs are put in there, and that a URL is always put in there.
BUG=
528228
TEST=existing tests
Committed:
https://crrev.com/7c0b59d9c9dfba08fe924dd0ee219ec94b65bb87
Cr-Commit-Position: refs/heads/master@{#351805}