Fix iOS 11 WebView extra white space on top due to contentInsetAdjustmentBehavior

Summary:
Same IOS 11 issue as https://github.com/facebook/react-native/pull/15023
Fixes the annoying 20px content insert on top.
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Fix the behaviour of WebView when building the iOS project with Xcode 9 due to the `contentInsetAdjustmentBehavior`.

![expected](https://user-images.githubusercontent.com/5630513/32207551-b7789668-bdca-11e7-840a-f325b2767d08.jpg)
![iphone_x_after](https://user-images.githubusercontent.com/5630513/32207557-beb8fd6e-bdca-11e7-87d0-18d533f20125.jpg)

![issue_normal_iphone_ios11](https://user-images.githubusercontent.com/5630513/32207572-d773be5c-bdca-11e7-8e28-8f0783eef1cd.jpg)
![iphone_x_before](https://user-images.githubusercontent.com/5630513/32207581-e3e93234-bdca-11e7-847d-f801bbf05d55.jpg)

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  CATEGORY
[----------]        TYPE
[ CLI      ]   [-------------]      LOCATION
[ DOCS     ]   [ BREAKING    ]   [-------------]
[ GENERAl  ]   [ BUGFIX      ]   [-{Component}-]
[ INTERNAL ]   [ ENHANCEMENT ]   [ {File}      ]
[ IOS      ]   [ FEATURE     ]   [ {Directory} ]   |-----------|
[ ANDROID  ]   [ MINOR       ]   [ {Framework} ] - | {Message} |
[----------]   [-------------]   [-------------]   |-----------|

[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] [BUGFIX] [WebView] - Fix extra white space added to webView due to iOS 11 contentInsetAdjustmentBehavior
Closes https://github.com/facebook/react-native/pull/16519

Differential Revision: D6195670

Pulled By: shergin

fbshipit-source-id: 08d4d4dc700059bb7707e038dc4f592af0275896

React/Views/RCTWebView.m

@@ -52,6 +52,11 @@ static NSString *const kPostMessageHost = @"postMessage";
     _contentInset = UIEdgeInsetsZero;
     _webView = [[UIWebView alloc] initWithFrame:self.bounds];
     _webView.delegate = self;
+#if defined(__IPHONE_OS_VERSION_MAX_ALLOWED) && __IPHONE_OS_VERSION_MAX_ALLOWED >= 110000 /* __IPHONE_11_0 */
+    if ([_webView.scrollView respondsToSelector:@selector(setContentInsetAdjustmentBehavior:)]) {
+      _webView.scrollView.contentInsetAdjustmentBehavior = UIScrollViewContentInsetAdjustmentNever;
+    }
+#endif
     [self addSubview:_webView];
   }
   return self;