CB-6837 Fix leaked window when hitting back button while alert being rendered

Keep track of the last AlertDialog showed.
The last dialog showed that is rendered while hitting back button it
causes a leaked window.
Instead of perform a full track of all dialogs created, only destroy the
last one showed, this fixes the problem.

close #122

framework/src/org/apache/cordova/CordovaChromeClient.java

@@ -65,6 +65,9 @@ public class CordovaChromeClient extends WebChromeClient {
     // the video progress view
     private View mVideoProgressView;
     
+    //Keep track of last AlertDialog showed
+    private AlertDialog lastHandledDialog;
+    
     // File Chooser
     public ValueCallback<Uri> mUploadMessage;
     
@@ -123,7 +126,7 @@ public class CordovaChromeClient extends WebChromeClient {
                     return true;
             }
         });
-        dlg.show();
+        lastHandledDialog = dlg.show();
         return true;
     }
 
@@ -172,7 +175,7 @@ public class CordovaChromeClient extends WebChromeClient {
                     return true;
             }
         });
-        dlg.show();
+        lastHandledDialog = dlg.show();
         return true;
     }
 
@@ -216,7 +219,7 @@ public class CordovaChromeClient extends WebChromeClient {
                             res.cancel();
                         }
                     });
-            dlg.show();
+            lastHandledDialog = dlg.show();
         }
         return true;
     }
@@ -328,4 +331,11 @@ public class CordovaChromeClient extends WebChromeClient {
     public ValueCallback<Uri> getValueCallback() {
         return this.mUploadMessage;
     }
+    
+    public void destroyLastDialog(){
+        if(lastHandledDialog != null){
+                lastHandledDialog.cancel();
+        }
+    }
+
 }

framework/src/org/apache/cordova/CordovaWebView.java

@@ -788,6 +788,9 @@ public class CordovaWebView extends WebView {
 
         // Load blank page so that JavaScript onunload is called
         this.loadUrl("about:blank");
+        
+        //Remove last AlertDialog
+        this.chromeClient.destroyLastDialog();
 
         // Forward to plugins
         if (this.pluginManager != null) {