Dynamically load WebKit

Summary:
@public

We can't dynamically link `WebKit` because doing so will impact cold start of all our Apps.

This diff includes a few changes:
 1. Weakly link the `WebKit` framework in the `ReactInternal` library, so that the compiler doesn't die when it encounters a WebKit symbol.
 2. Undo dynamic linking of WebKit in Catalyst.
 3. Undo dynamic linking of WebKit in AdsManager
 4. Before the first `WKWebView` is instantiated, dynamically load the `WebKit` framework.

The end result of these changes is that WebKit will be loaded only when it's going to be used.

Reviewed By: mmmulani

Differential Revision: D6564328

fbshipit-source-id: a45a44e774d0c61c1fb578a6fa3d16bb08f68ac9

Author: RSNara

React/Base/RCTConvert.h

@@ -69,6 +69,9 @@ typedef NSURL RCTFileURL;
 + (UIReturnKeyType)UIReturnKeyType:(id)json;
 #if !TARGET_OS_TV
 + (UIDataDetectorTypes)UIDataDetectorTypes:(id)json;
+#endif
+
+#if TARGET_OS_IPHONE
 + (WKDataDetectorTypes)WKDataDetectorTypes:(id)json;
 #endif
 

React/React.xcodeproj/project.pbxproj

@@ -5240,7 +5240,11 @@
 					"RCT_METRO_PORT=${RCT_METRO_PORT}",
 				);
 				GCC_WARN_ABOUT_MISSING_NEWLINE = YES;
-				OTHER_LDFLAGS = "-ObjC";
+				OTHER_LDFLAGS = (
+					"-ObjC",
+					"-weak_framework",
+					WebKit,
+				);
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				PUBLIC_HEADERS_FOLDER_PATH = /usr/local/include/React;
 				RUN_CLANG_STATIC_ANALYZER = YES;
@@ -5258,7 +5262,11 @@
 					"RCT_METRO_PORT=${RCT_METRO_PORT}",
 				);
 				GCC_WARN_ABOUT_MISSING_NEWLINE = YES;
-				OTHER_LDFLAGS = "-ObjC";
+				OTHER_LDFLAGS = (
+					"-ObjC",
+					"-weak_framework",
+					WebKit,
+				);
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				PUBLIC_HEADERS_FOLDER_PATH = /usr/local/include/React;
 				RUN_CLANG_STATIC_ANALYZER = NO;

React/Views/RCTWKWebView.m

@@ -23,6 +23,25 @@ - (void)dealloc
 
 }
 
+/**
+ * See https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/DisplayWebContent/Tasks/WebKitAvail.html.
+ */
++ (BOOL)dynamicallyLoadWebKitIfAvailable
+{
+  static BOOL _webkitAvailable=NO;
+  static dispatch_once_t onceToken;
+
+  dispatch_once(&onceToken, ^{
+    NSBundle *webKitBundle = [NSBundle bundleWithPath:@"/System/Library/Frameworks/WebKit.framework"];
+    if (webKitBundle) {
+      _webkitAvailable = [webKitBundle load];
+    }
+  });
+
+  return _webkitAvailable;
+}
+
+
 - (instancetype)initWithFrame:(CGRect)frame
 {
   if ((self = [super initWithFrame:frame])) {
@@ -38,6 +57,10 @@ - (instancetype)initWithFrame:(CGRect)frame
 - (void)didMoveToWindow
 {
   if (self.window != nil) {
+    if (![[self class] dynamicallyLoadWebKitIfAvailable]) {
+      return;
+    };
+
     WKWebViewConfiguration *wkWebViewConfig = [WKWebViewConfiguration new];
     wkWebViewConfig.userContentController = [WKUserContentController new];
     [wkWebViewConfig.userContentController addScriptMessageHandler: self name: MessageHanderName];