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In preparation of componentization of FaviconTabHelper, introduce a
new target //components/favicon/content (as favicon is a layered
component) and move the function to convert from content::FaviconURL
to favicon::FaviconURL there.

BUG=374281

Review URL: https://codereview.chromium.org/1020213002

Cr-Commit-Position: refs/heads/master@{#321606}