fix(iOS): inconsistent backgroundColor (#868)

* fix inconsistent backgroundColor

* remove redundant setBackgroundColor

ios/RNCWebView.m

     }
 
     _webView = [[WKWebView alloc] initWithFrame:self.bounds configuration: wkWebViewConfig];
+    [self setBackgroundColor: _savedBackgroundColor];
     _webView.scrollView.delegate = self;
     _webView.UIDelegate = self;
     _webView.navigationDelegate = self;
     }];
     _onLoadingError(event);
   }
-
-  [self setBackgroundColor: _savedBackgroundColor];
 }
 
 - (void)evaluateJS:(NSString *)js
   } else if (_onLoadingFinish) {
     _onLoadingFinish([self baseEvent]);
   }
-
-  [self setBackgroundColor: _savedBackgroundColor];
 }
 
 - (void)injectJavaScript:(NSString *)script