Implement 'goForward' and 'goBack' methods

Summary:
@public

This diff implements the `goForward` and `goBack` methods on `WKWebView`.
1. `goForward` moves the web view one screen forward in the browser history.
1. `goBack` moves the web view one screen back in the browser history.

Reviewed By: shergin

Differential Revision: D6367495

fbshipit-source-id: e100ca00e92a6eaa30d2af1af642ba79a9c9feae

Author: RSNara

React/Views/RCTWKWebView.h

@@ -25,5 +25,7 @@
 
 - (void)postMessage:(NSString *)message;
 - (void)injectJavaScript:(NSString *)script;
+- (void)goForward;
+- (void)goBack;
 
 @end

React/Views/RCTWKWebView.m

@@ -260,4 +260,14 @@ - (void)injectJavaScript:(NSString *)script
   [self evaluateJS: script thenCall: nil];
 }
 
+- (void)goForward
+{
+  [_webView goForward];
+}
+
+- (void)goBack
+{
+  [_webView goBack];
+}
+
 @end

React/Views/RCTWKWebViewManager.m

@@ -57,4 +57,28 @@ - (UIView *)view
   }];
 }
 
+RCT_EXPORT_METHOD(goBack:(nonnull NSNumber *)reactTag)
+{
+  [self.bridge.uiManager addUIBlock:^(__unused RCTUIManager *uiManager, NSDictionary<NSNumber *, RCTWKWebView *> *viewRegistry) {
+    RCTWKWebView *view = viewRegistry[reactTag];
+    if (![view isKindOfClass:[RCTWKWebView class]]) {
+      RCTLogError(@"Invalid view returned from registry, expecting RCTWebView, got: %@", view);
+    } else {
+      [view goBack];
+    }
+  }];
+}
+
+RCT_EXPORT_METHOD(goForward:(nonnull NSNumber *)reactTag)
+{
+  [self.bridge.uiManager addUIBlock:^(__unused RCTUIManager *uiManager, NSDictionary<NSNumber *, RCTWKWebView *> *viewRegistry) {
+    RCTWKWebView *view = viewRegistry[reactTag];
+    if (![view isKindOfClass:[RCTWKWebView class]]) {
+      RCTLogError(@"Invalid view returned from registry, expecting RCTWebView, got: %@", view);
+    } else {
+      [view goForward];
+    }
+  }];
+}
+
 @end