Bug 1356826: Part 6 - Wait for delayed startup before checking for side-loads. r=aswan

MozReview-Commit-ID: IpuRNbdUXmE

--HG--
extra : rebase_source : 5ac9b960f9f06952120676b713d77624f832e4d6

browser/base/content/browser.js

@@ -1143,6 +1143,11 @@ addEventListener("DOMContentLoaded", function onDCL() {
   gBrowser.updateBrowserRemoteness(initBrowser, gMultiProcessBrowser);
 });
 
+let _resolveDelayedStartup;
+var delayedStartupPromise = new Promise(resolve => {
+  _resolveDelayedStartup = resolve;
+});
+
 var gBrowserInit = {
   delayedStartupFinished: false,
 
@@ -1619,6 +1624,7 @@ var gBrowserInit = {
 
     this.delayedStartupFinished = true;
 
+    _resolveDelayedStartup();
     Services.obs.notifyObservers(window, "browser-delayed-startup-finished");
     TelemetryTimestamps.add("delayedStartupFinished");
   },

browser/modules/ExtensionsUI.jsm

@@ -35,7 +35,7 @@ this.ExtensionsUI = {
   sideloadListener: null,
   histogram: null,
 
-  init() {
+  async init() {
     this.histogram = Services.telemetry.getHistogramById("EXTENSION_INSTALL_PROMPT_RESULT");
 
     Services.obs.addObserver(this, "webextension-permission-prompt");
@@ -43,6 +43,8 @@ this.ExtensionsUI = {
     Services.obs.addObserver(this, "webextension-install-notify");
     Services.obs.addObserver(this, "webextension-optional-permission-prompt");
 
+    await RecentWindow.getMostRecentBrowserWindow().delayedStartupPromise;
+
     this._checkForSideloaded();
   },