Implement 'decelerationRate' prop

Summary:
@public

The content that renders within the `WKWebView` instance actually renders inside a `UIScrollView`. On that scroll view, we can adjust the `decelerationRate`, which controls how fast the view stops scrolling after the user lets go while scrolling.

In this diff, I implemented the `decelerationRate` prop for `WKWebView`, which gets forwarded to the `UIScrollView` instance underlying the web view.

**Note:** Even though we accept a floating point value for the deceleration rate, the native `UIScrollView` component only allows two inputs:
1. `UIScrollViewDecelerationRateNormal`: 0.998
2. `UIScrollViewDecelerationRateFast`: 0.99

As far as I know, it seems to just round up to the nearest valid `CGFloat` (or down if number > 0.998), for any invalid numbers.

Reviewed By: mmmulani

Differential Revision: D6307262

fbshipit-source-id: 98c4395702415aa36519f9e9bd84f043be3a5881

React/Views/RCTWKWebView.h

@@ -21,6 +21,7 @@
 @property (nonatomic, assign) BOOL messagingEnabled;
 @property (nonatomic, copy) NSString *injectedJavaScript;
 @property (nonatomic, assign) BOOL scrollEnabled;
+@property (nonatomic, assign) CGFloat decelerationRate;
 
 - (void)postMessage:(NSString *)message;
 

React/Views/RCTWKWebView.m

@@ -5,7 +5,7 @@
 
 static NSString *const MessageHanderName = @"ReactNative";
 
-@interface RCTWKWebView () <WKUIDelegate, WKNavigationDelegate, WKScriptMessageHandler>
+@interface RCTWKWebView () <WKUIDelegate, WKNavigationDelegate, WKScriptMessageHandler, UIScrollViewDelegate>
 @property (nonatomic, copy) RCTDirectEventBlock onLoadingStart;
 @property (nonatomic, copy) RCTDirectEventBlock onLoadingFinish;
 @property (nonatomic, copy) RCTDirectEventBlock onLoadingError;
@@ -31,6 +31,7 @@ - (instancetype)initWithFrame:(CGRect)frame
     [wkWebViewConfig.userContentController addScriptMessageHandler: self name: MessageHanderName];
 
     _webView = [[WKWebView alloc] initWithFrame:self.bounds configuration: wkWebViewConfig];
+    _webView.scrollView.delegate = self;
     _webView.UIDelegate = self;
     _webView.navigationDelegate = self;
     [self addSubview:_webView];
@@ -85,6 +86,12 @@ - (void)setSource:(NSDictionary *)source
   }
 }
 
+
+- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
+{
+  scrollView.decelerationRate = _decelerationRate;
+}
+
 - (void)setScrollEnabled:(BOOL)scrollEnabled
 {
   _webView.scrollView.scrollEnabled = scrollEnabled;

React/Views/RCTWKWebViewManager.m

@@ -41,4 +41,8 @@ - (UIView *)view
   view.scrollEnabled = json == nil ? true : [RCTConvert BOOL: json];
 }
 
+RCT_CUSTOM_VIEW_PROPERTY(decelerationRate, CGFloat, RCTWKWebView) {
+  view.decelerationRate = json == nil ? UIScrollViewDecelerationRateNormal : [RCTConvert CGFloat: json];
+}
+
 @end