Adds _lineHeight = NAN; to RCTTextAttributes

Summary:
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On iOS, when the parent Text contains the `lineHeight` style prop, and the children are also Text components they don't inherit the lineHeight prop.

This is for **iOS** only.
Create a react-native project with React Native 0.54.0 or 0.54.1 and change the app to:
```js
import React, { Component } from 'react';
import { StyleSheet, Text, View } from 'react-native';

type Props = {};
export default class App extends Component<Props> {
  render() {
    return (
      <View style={styles.container}>
        <Text style={{ fontWeight: 'bold', lineHeight: 40 }}>
          <Text style={{ color: 'orange' }}>I am bold and orange, </Text>
          <Text style={{ color: 'red' }}>I am bold and red, </Text>
          <Text style={{ color: 'blue' }}>I am bold and blue, </Text>
          <Text style={{ color: 'purple' }}>I am bold and purple, </Text>
          <Text style={{ color: 'yellow' }}>I am bold and yellow, </Text>
          <Text style={{ color: 'pink' }}>I am bold and pink</Text>
        </Text>
      </View>
    );
  }
}

const styles = StyleSheet.create({
  container: {
    backgroundColor: 'black',
    flex: 1,
    justifyContent: 'center',
    padding: 50,
  },
});
```
It displays:
![iphone - 2018-03-12 at 21 51 53](https://user-images.githubusercontent.com/480605/37308941-b56f082e-263f-11e8-9c23-892f77077169.png)

But should look like:
![iphone - 2018-03-12 at 21 48 15](https://user-images.githubusercontent.com/480605/37308784-4efaddf2-263f-11e8-992b-ee0b6bb9a97b.png)

New &lt;Text> iOS 2716f53

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  CATEGORY
[----------]        TYPE
[ CLI      ]   [-------------]      LOCATION
[ DOCS     ]   [ BREAKING    ]   [-------------]
[ GENERAL  ]   [ BUGFIX      ]   [-{Component}-]
[ INTERNAL ]   [ ENHANCEMENT ]   [ {File}      ]
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[----------]   [-------------]   [-------------]   |-----------|

[CATEGORY] [TYPE] [LOCATION] - MESSAGE

 EXAMPLES:

 [IOS] [BREAKING] [FlatList] - Change a thing that breaks other things
 [ANDROID] [BUGFIX] [TextInput] - Did a thing to TextInput
 [CLI] [FEATURE] [local-cli/info/info.js] - CLI easier to do things with
 [DOCS] [BUGFIX] [GettingStarted.md] - Accidentally a thing/word
 [GENERAL] [ENHANCEMENT] [Yoga] - Added new yoga thing/position
 [INTERNAL] [FEATURE] [./scripts] - Added thing to script that nobody will see
-->
[IOS] [MINOR] [Text] - Inherit lineheight
Closes #18340

Differential Revision: D7276655

Pulled By: shergin

fbshipit-source-id: 0fe26536bb74da77be8405911fc699a622bc0b2f

Libraries/Text/RCTTextAttributes.m

@@ -21,6 +21,7 @@ - (instancetype)init
   if (self = [super init]) {
     _fontSize = NAN;
     _letterSpacing = NAN;
+    _lineHeight = NAN;
     _textDecorationStyle = NSUnderlineStyleSingle;
     _fontSizeMultiplier = NAN;
     _alignment = NSTextAlignmentNatural;