(iOS & Android) Add postMessage API support (#362)

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### Platforms affected
Android
iOS (both UIWebView & WKWebView implementations)

### What does this PR do?
Adds support for [postMessage API](https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage) enabling pages loaded into the InappBrowser to post messages back to the parent Webview of the Cordova app.

For example, sending event messages associated with UI interactions such as button clicks from the wrapped page back to the parent app Webview.

### What testing has been done on this change?
Automated tests have been extended to cover the `message` event.

### Checklist
- [x ] Commit message follows the format: "GH-3232: (android) Fix bug with resolving file paths", where CB-xxxx is the JIRA ID & "android" is the platform affected.
- [ x] Added automated test coverage as appropriate for this change.

src/android/InAppBrowser.java

@@ -48,6 +48,7 @@ import android.view.inputmethod.InputMethodManager;
 import android.webkit.CookieManager;
 import android.webkit.CookieSyncManager;
 import android.webkit.HttpAuthHandler;
+import android.webkit.JavascriptInterface;
 import android.webkit.ValueCallback;
 import android.webkit.WebChromeClient;
 import android.webkit.WebSettings;
@@ -95,6 +96,7 @@ public class InAppBrowser extends CordovaPlugin {
     private static final String LOAD_START_EVENT = "loadstart";
     private static final String LOAD_STOP_EVENT = "loadstop";
     private static final String LOAD_ERROR_EVENT = "loaderror";
+    private static final String MESSAGE_EVENT = "message";
     private static final String CLEAR_ALL_CACHE = "clearcache";
     private static final String CLEAR_SESSION_CACHE = "clearsessioncache";
     private static final String HARDWARE_BACK_BUTTON = "hardwareback";
@@ -952,8 +954,24 @@ public class InAppBrowser extends CordovaPlugin {
                 settings.setBuiltInZoomControls(showZoomControls);
                 settings.setPluginState(android.webkit.WebSettings.PluginState.ON);
 
+                // Add postMessage interface
+                class JsObject {
+                    @JavascriptInterface
+                    public void postMessage(String data) {
+                        try {
+                            JSONObject obj = new JSONObject();
+                            obj.put("type", MESSAGE_EVENT);
+                            obj.put("data", new JSONObject(data));
+                            sendUpdate(obj, true);
+                        } catch (JSONException ex) {
+                            LOG.e(LOG_TAG, "data object passed to postMessage has caused a JSON error.");
+                        }
+                    }
+                }
+
                 if(android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.JELLY_BEAN_MR1) {
                     settings.setMediaPlaybackRequiresUserGesture(mediaPlaybackRequiresUserGesture);
+                    inAppWebView.addJavascriptInterface(new JsObject(), "cordova_iab");
                 }
 
                 String overrideUserAgent = preferences.getString("OverrideUserAgent", null);
@@ -1270,6 +1288,11 @@ public class InAppBrowser extends CordovaPlugin {
         public void onPageFinished(WebView view, String url) {
             super.onPageFinished(view, url);
 
+            // Set the namespace for postMessage()
+            if (Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.JELLY_BEAN_MR1){
+                injectDeferredObject("window.webkit={messageHandlers:{cordova_iab:cordova_iab}}", null);
+            }
+
             // CB-10395 InAppBrowser's WebView not storing cookies reliable to local device storage
             if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.LOLLIPOP) {
                 CookieManager.getInstance().flush();