zlib/pdf: remove HaveFlate(), depend on preprocessor defines

gyp/skflate.gyp

 # Target for including SkFlate.
 {
   'targets': [
     {
       'target_name': 'skflate',
       'type': 'static_library',
       'dependencies': [
         'skia_lib.gyp:skia_lib',
       ],
       'conditions': [
         # When zlib is not availible on a system,
-        # SkFlate::HaveFlate will just return false.
+        # SK_NO_FLATE will be defined.
         [ 'skia_os != "win"',
           {
             'dependencies': [
               'zlib.gyp:zlib',
             ],
           }
         ],
       ],
       'sources': [
         '../src/core/SkFlate.cpp',
       ],
     },
   ],
 }