Only accept HTTP and HTTPS urls for opensearch descriptor.
This CL prevents http: or https: urls from referring to file: urls as search descriptor xmls.
Allowing urls to refer to other urls with the same scheme has been considered (e.g. a file: url
referring to an OSDD xml from a file: url), but this currently doesn't work for file: urls
because of http://b/863583, so is not implemented here.
BUG=429838
Review URL: https://codereview.chromium.org/
917313004
Cr-Commit-Position: refs/heads/master@{#317723}