Simplify flattening to just write enough to call the factory

src/opts/SkXfermode_opts_arm_neon.h

 #ifndef SkXfermode_opts_arm_neon_DEFINED
 #define SkXfermode_opts_arm_neon_DEFINED
 
 #include "SkXfermode_proccoeff.h"
 
 class SkNEONProcCoeffXfermode : public SkProcCoeffXfermode {
 public:
     SkNEONProcCoeffXfermode(const ProcCoeff& rec, SkXfermode::Mode mode,
                             void* procSIMD)
             : INHERITED(rec, mode), fProcSIMD(procSIMD) {}
 
     virtual void xfer32(SkPMColor dst[], const SkPMColor src[], int count,
                         const SkAlpha aa[]) const SK_OVERRIDE;
     virtual void xfer16(uint16_t* SK_RESTRICT dst, const SkPMColor* SK_RESTRICT src,
                         int count, const SkAlpha* SK_RESTRICT aa) const SK_OVERRIDE;
 
     SK_TO_STRING_OVERRIDE()
-    SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkNEONProcCoeffXfermode)
 
 private:
+#ifdef SK_SUPPORT_LEGACY_DEEPFLATTENING
     SkNEONProcCoeffXfermode(SkReadBuffer& buffer);
+#endif
 
     // void* is used to avoid pulling arm_neon.h in the core and having to build
     // it with -mfpu=neon.
     void* fProcSIMD;
     typedef SkProcCoeffXfermode INHERITED;
 };
 
 extern SkPMColor srcatop_modeproc_neon(SkPMColor src, SkPMColor dst);
 extern SkPMColor dstatop_modeproc_neon(SkPMColor src, SkPMColor dst);
 extern SkPMColor xor_modeproc_neon(SkPMColor src, SkPMColor dst);
 extern SkPMColor plus_modeproc_neon(SkPMColor src, SkPMColor dst);
 extern SkPMColor modulate_modeproc_neon(SkPMColor src, SkPMColor dst);
 
 #endif //#ifdef SkXfermode_opts_arm_neon_DEFINED