Bug 1181601 - Make sure RemoteWebProgress is instantiated for preloaded browsers. r=ttaubert

When remoting the about:newtab page, we noticed that the parent still thought that
the preloaded about:newtab browsers were all pointed at about:blank still. This
was because the preloaded browsers didn't have their RemoteWebProgress's
initialized in time to receive the location change messages from the child.

--HG--
extra : commitid : CzPtTYPvHCg
extra : rebase_source : 5bf11e9f4fcd0dc03acedb538c3eac6110a9182c
extra : amend_source : 6d4d8c21cdeabb936a5044b0301f68adb0e82441

browser/base/content/tabbrowser.xml

@@ -1631,6 +1631,13 @@
             let notificationbox = this.getNotificationBox(browser);
             this.mPanelContainer.appendChild(notificationbox);
 
+            if (remote) {
+              // For remote browsers, we need to make sure that the webProgress is
+              // instantiated, otherwise the parent won't get informed about the state
+              // of the preloaded browser until it gets attached to a tab.
+              browser.webProgress;
+            }
+
             browser.loadURI(BROWSER_NEW_TAB_URL);
             browser.docShellIsActive = false;
           ]]>