Modify FormPanel for compliance with Strict CSP

More detail on Strict CSP:
https://csp.withgoogle.com/docs/strict-csp.html

Change-Id: I24fdf2e1c9c251d7b117ff8fa21e805b30a74c36
Review-Link: https://gwt-review.googlesource.com/#/c/17740/
diff --git a/user/src/com/google/gwt/user/client/ui/FormPanel.java b/user/src/com/google/gwt/user/client/ui/FormPanel.java
index 0a1c0c0..396f8d6 100644
--- a/user/src/com/google/gwt/user/client/ui/FormPanel.java
+++ b/user/src/com/google/gwt/user/client/ui/FormPanel.java
@@ -214,7 +214,7 @@
   interface IFrameTemplate extends SafeHtmlTemplates {
     static final IFrameTemplate INSTANCE = GWT.create(IFrameTemplate.class);
 
-    @Template("<iframe src=\"javascript:''\" name='{0}' tabindex='-1' "
+    @Template("<iframe src=\"about:blank\" name='{0}' tabindex='-1' "
         + "style='position:absolute;width:0;height:0;border:0'>")
     SafeHtml get(String name);
   }