Merge to XFA: Move FPDFAPI_FlateInit() prototype and friends to .h file

(cherry picked from commit dacc22cdfea727a04bce086d9bfec9a4d1a29bd4)
Orignal Review URL: https://codereview.chromium.org/1240713004 .
TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/1230373003 .
5 files changed