[Style] added backface-visibility. fixes #1973

Summary:
default behavior remains same(backface visible).

cc @sahrens
Closes https://github.com/facebook/react-native/pull/1977
Github Author: chirag04 <jain_chirag04@yahoo.com>

React/Base/RCTConvert.h

@@ -120,8 +120,9 @@ typedef NSArray CGColorArray;
 typedef id NSPropertyList;
 + (NSPropertyList)NSPropertyList:(id)json;
 
-typedef BOOL css_clip_t;
+typedef BOOL css_clip_t, css_backface_visibility_t;
 + (css_clip_t)css_clip_t:(id)json;
++ (css_backface_visibility_t)css_backface_visibility_t:(id)json;
 + (css_flex_direction_t)css_flex_direction_t:(id)json;
 + (css_justify_t)css_justify_t:(id)json;
 + (css_align_t)css_align_t:(id)json;

React/Base/RCTConvert.m

@@ -993,6 +993,11 @@ static id RCTConvertPropertyListValue(id json)
   return RCTConvertPropertyListValue(json);
 }
 
+RCT_ENUM_CONVERTER(css_backface_visibility_t, (@{
+  @"hidden": @NO,
+  @"visible": @YES
+}), YES, boolValue)
+
 RCT_ENUM_CONVERTER(css_clip_t, (@{
   @"hidden": @YES,
   @"visible": @NO