Bug 1526822 - Fix headless screenshot that has redirects. r=mossop

The changes in bug 1507352 caused onLocationChange events to be ignored
when there was an http redirect. This caused the screenshot command to hang
since it never attached an event listener on the page it was redirected to.

Differential Revision: https://phabricator.services.mozilla.com/D22220

--HG--
extra : moz-landing-system : lando

browser/components/shell/HeadlessShell.jsm

@@ -41,8 +41,8 @@ function loadContentWindow(webNavigation, url, principal) {
         if (flags & Ci.nsIWebProgressListener.LOCATION_CHANGE_SAME_DOCUMENT) {
           return;
         }
-        // Ignore the initial about:blank
-        if (uri.spec != location.spec) {
+        // Ignore the initial about:blank, unless about:blank is requested
+        if (location.spec == "about:blank" && uri.spec != "about:blank") {
           return;
         }
         let contentWindow = docShell.domWindow;

browser/components/shell/test/chrome.ini

@@ -1,5 +1,8 @@
 [DEFAULT]
-support-files = headless.html
+support-files =
+  headless.html
+  headless_redirect.html
+  headless_redirect.html^headers^
 
 [test_headless_screenshot.html]
 skip-if = toolkit == 'android'

browser/components/shell/test/headless_redirect.html^headers^

@@ -0,0 +1,2 @@
+HTTP 302 Moved Temporarily
+Location: headless.html

browser/components/shell/test/test_headless_screenshot.html

@@ -137,6 +137,9 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=1378010
     await testFileCreationNegative(["-url", "http://mochi.test:8888/chrome/browser/components/shell/test/headless.html", "-screenshot", "-window-size", "hello"], "screenshot.png");
     await testFileCreationNegative(["-url", "http://mochi.test:8888/chrome/browser/components/shell/test/headless.html", "-screenshot", "-window-size", "800,"], "screenshot.png");
 
+    // Test when the requested URL redirects
+    await testFileCreationPositive(["-url", "http://mochi.test:8888/chrome/browser/components/shell/test/headless_redirect.html", "-screenshot", screenshotPath], screenshotPath);
+
     SimpleTest.finish();
   })();
   </script>