Add baseline alignment support

Summary:
Add baseline alignment support to react native.

{F65372439}

```
class Playground extends React.Component {
  render() {
    return (
      <View style={{padding: 30, flexDirection: 'row', alignItems: 'baseline'}}>
        <View style={{width: 30, height: 10, backgroundColor: 'red'}}/>
        <View style={{width: 30, height: 20, backgroundColor: 'green'}}/>
        <View style={{width: 30, height: 30, backgroundColor: 'blue'}}/>
      </View>
    );
  }
}
```

Reviewed By: javache

Differential Revision: D4385099

fbshipit-source-id: d7caa6e4c086c4a62e24ef1d5db9c805c470ef2a

Libraries/StyleSheet/LayoutPropTypes.js

@@ -311,7 +311,8 @@ var LayoutPropTypes = {
     'flex-start',
     'flex-end',
     'center',
-    'stretch'
+    'stretch',
+    'baseline'
   ]),
 
   /** `alignSelf` controls how a child aligns in the cross direction,
@@ -325,7 +326,8 @@ var LayoutPropTypes = {
     'flex-start',
     'flex-end',
     'center',
-    'stretch'
+    'stretch',
+    'baseline'
   ]),
 
   /** `overflow` controls how a children are measured and displayed.

React/Base/RCTConvert.m

@@ -640,7 +640,8 @@ + (NSPropertyList)NSPropertyList:(id)json
   @"flex-end": @(YGAlignFlexEnd),
   @"center": @(YGAlignCenter),
   @"auto": @(YGAlignAuto),
-  @"stretch": @(YGAlignStretch)
+  @"stretch": @(YGAlignStretch),
+  @"baseline": @(YGAlignBaseline)
 }), YGAlignFlexStart, intValue)
 
 RCT_ENUM_CONVERTER(YGPositionType, (@{