[WebView]: Kill shouldInjectAJAXHandler, and add injectedJavascriptIOS

Summary:
@public

The API and implementation of `shouldInjectAJAXHandler` is very opinionated, and it does not solve many of the use cases that we'd like to address.

Since  `shouldInjectAJAXHandler` is basically juts injecting JS to the web page, we should let developer inject whatever JS that address different issues that they want to fix.

Test Plan:
Test this snippet at <Playground />

```
<WebView
  url="http://www.facebook.com"
  injectedJavascriptIOS="document.body.style.border='solid 10px red'"
/>
```

Libraries/Components/WebView/WebView.ios.js

@@ -43,6 +43,8 @@ var NavigationType = {
   other: RCTWebViewManager.NavigationType.Other,
 };
 
+var JSNavigationScheme = RCTWebViewManager.JSNavigationScheme;
+
 type ErrorEvent = {
   domain: any;
   code: any;
@@ -75,6 +77,7 @@ var defaultRenderError = (errorDomain, errorCode, errorDesc) => (
 
 var WebView = React.createClass({
   statics: {
+    JSNavigationScheme: JSNavigationScheme,
     NavigationType: NavigationType,
   },
 
@@ -86,7 +89,6 @@ var WebView = React.createClass({
     bounces: PropTypes.bool,
     scrollEnabled: PropTypes.bool,
     automaticallyAdjustContentInsets: PropTypes.bool,
-    shouldInjectAJAXHandler: PropTypes.bool,
     contentInset: EdgeInsetsPropType,
     onNavigationStateChange: PropTypes.func,
     startInLoadingState: PropTypes.bool, // force WebView to show loadingView on first load
@@ -95,6 +97,11 @@ var WebView = React.createClass({
      * Used for android only, JS is enabled by default for WebView on iOS
      */
     javaScriptEnabledAndroid: PropTypes.bool,
+    /**
+     * Used for iOS only, sets the JS to be injected when the webpage loads.
+     */
+    injectedJavascriptIOS: PropTypes.string,
+
     /**
      * Used for iOS only, sets whether the webpage scales to fit the view and the
      * user can change the scale
@@ -152,9 +159,9 @@ var WebView = React.createClass({
         style={webViewStyles}
         url={this.props.url}
         html={this.props.html}
+        injectedJavascriptIOS={this.props.injectedJavascriptIOS}
         bounces={this.props.bounces}
         scrollEnabled={this.props.scrollEnabled}
-        shouldInjectAJAXHandler={this.props.shouldInjectAJAXHandler}
         contentInset={this.props.contentInset}
         automaticallyAdjustContentInsets={this.props.automaticallyAdjustContentInsets}
         onLoadingStart={this.onLoadingStart}