Re-virtualize CFX_UnicodeEncoding.
Re-virtualize CFX_UnicodeEncoding.

It's not required on master, but it costs very little and
makes XFA line up more closely. Also make InitFTLibrary()
public for the same reasons.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1475643003 .

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