DevTools: [Documentation] Disable external XHR requests in documentation tests

LayoutTests/inspector/documentation/documentation-test.js

+function initialize_DocumentationTests()
+{
+
+InspectorTest.mockLoadXHR = function(urlPrefix, responseText)
+{
+    window.loadXHR = function(url)
+    {
+        return new Promise(load);

[lushnikov, 2014/09/12 10:47:20]

you want to discretely fall-back to the original loadXHR method if the url here
doesn't match with urlPrefix.

This will avoid you from copy-pasting loadXHR method in here.

+
+        function load(successCallback, failureCallback)
+        {
+            function onReadyStateChanged()
+            {
+                if (xhr.readyState !== XMLHttpRequest.DONE)
+                    return;
+                if (xhr.status !== 200) {
+                    xhr.onreadystatechange = null;
+                    failureCallback(new Error(xhr.status));
+                    return;
+                }
+                xhr.onreadystatechange = null;
+                successCallback(xhr.responseText);
+            }
+
+            if (url.startsWith(urlPrefix)) {
+                successCallback(responseText);
+                return;
+            }
+
+            var xhr = new XMLHttpRequest();
+            xhr.open("GET", url, true);
+            xhr.onreadystatechange = onReadyStateChanged;
+            xhr.send(null);
+        }
+    }
+}
+
+}