bug #14170 Fix the AJAX profiling (stof)

This PR was merged into the 2.6 branch.

Discussion
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Fix the AJAX profiling

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | reported in https://github.com/symfony/symfony/issues/13447#issuecomment-88849938
| License       | MIT
| Doc PR        | n/a

The fix for IE8 (#13978) which does not have the addEventListener method on XMLHttpRequest broke the feature for modern browsers because it was checking the existence on the wrong object. It is a method on the instance, not on the "class", and so should be checked on the prototype.

Commits
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9d6c0b1 Fix the AJAX profiling

src/Symfony/Bundle/WebProfilerBundle/Resources/views/Profiler/base_js.html.twig

@@ -196,7 +196,7 @@
         }
 
         {% if excluded_ajax_paths is defined %}
-            if (window.XMLHttpRequest && XMLHttpRequest.addEventListener) {
+            if (window.XMLHttpRequest && XMLHttpRequest.prototype.addEventListener) {
                 var proxied = XMLHttpRequest.prototype.open;
 
                 XMLHttpRequest.prototype.open = function(method, url, async, user, pass) {