Bug 1588193 - Don't dispatch AboutNetErrorLoad until we're completely setup, r=johannh

BrowserTestUtils.waitForErrorPage may resolve slightly earlier than it did
before, so we may arrive at an about:neterror page that hasn't been completely
initialized. We should only dispatch the AboutNetErrorLoad event when we're done
making changes to the page.

Differential Revision: https://phabricator.services.mozilla.com/D51439

browser/base/content/aboutNetError.js

@@ -265,10 +265,6 @@ function initPage() {
     document.getElementById("netErrorButtonContainer").style.display = "none";
   }
 
-  // Dispatch this event only for tests.
-  let event = new CustomEvent("AboutNetErrorLoad", { bubbles: true });
-  document.dispatchEvent(event);
-
   setNetErrorMessageFromCode();
   let learnMoreLink = document.getElementById("learnMoreLink");
   let baseURL = RPMGetFormatURLPref("app.support.baseURL");
@@ -325,6 +321,11 @@ function initPage() {
       span.textContent = document.location.hostname;
     }
   }
+
+  // Dispatch this event only for tests. This should only be sent after we're
+  // done initializing the error page.
+  let event = new CustomEvent("AboutNetErrorLoad", { bubbles: true });
+  document.dispatchEvent(event);
 }
 
 function setupErrorUI() {