Implement the logic to find the correct <link> for Manifest in Document.

This is will then be use by WebFrame to get the data from the
HTMLLinkElement and fetch the ressource.

BUG=366145

Review URL: https://codereview.chromium.org/270283007

git-svn-id: svn://svn.chromium.org/blink/trunk@173823 bbb929c8-8fbe-4397-9dbb-9b2b20218538