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This CL is lifting out the FcPattern accessors to the fontconfig_util
file. It is also adding the unittests related to that code.

The goal of this refactoring is to share the accessors with the new
fallback font caching algorithm.
R=robliao@chromium.org

Bug: 1008531
Change-Id: Ib8d0ec9ef2ec93b91247595f7b03613d640ade80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1830059
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701317}