Way to pass styles to nested view

Summary:
Ability to pass styles to nested view for _position_ behavior.

Use case:

``` jsx
<KeyboardAvoidingView
          behavior={'position'}
          style={{ flex: 1 }}
          contentContainerStyle={{ flex: 1 }}
          keyboardVerticalOffset={ - VERTICAL_OFFSET }
>
          <SomeContainer   />
</KeyboardAvoidingView>
```
Closes https://github.com/facebook/react-native/pull/9065

Differential Revision: D3662876

Pulled By: javache

fbshipit-source-id: 9b08a04449431c4e563ef4464b5a7dba1fc02e4b

Libraries/Components/Keyboard/KeyboardAvoidingView.js

@@ -54,6 +54,11 @@ const KeyboardAvoidingView = React.createClass({
     ...View.propTypes,
     behavior: PropTypes.oneOf(['height', 'position', 'padding']),
     
+    /**
+     * The style of the content container(View) when behavior is 'position'.
+     */
+    contentContainerStyle: View.propTypes.style,
+    
     /**
      * This is the distance between the top of the user screen and the react native view,
      * may be non-zero in some use cases.
@@ -160,9 +165,11 @@ const KeyboardAvoidingView = React.createClass({
 
       case 'position':
         const positionStyle = {bottom: this.state.bottom};
+        const { contentContainerStyle } = this.props;
+
         return (
           <View ref={viewRef} style={style} onLayout={this.onLayout} {...props}>
-            <View style={positionStyle}>
+            <View style={[contentContainerStyle, positionStyle]}>
               {children}
             </View>
           </View>