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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}