Bug 1499661 - use 'this' instead of 'gBrowser' in moveTabsToStart and moveTabsToEnd, r=dao

Differential Revision: https://phabricator.services.mozilla.com/D9374

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

PHABTEST

@@ -0,0 +1 @@
+change

browser/base/content/tabbrowser.js

@@ -2580,7 +2580,7 @@ window._gBrowser = {
 
   moveTabsToStart(contextTab) {
     let tabs = contextTab.multiselected ?
-      gBrowser.selectedTabs :
+      this.selectedTabs :
       [contextTab];
     // Walk the array in reverse order so the tabs are kept in order.
     for (let i = tabs.length - 1; i >= 0; i--) {
@@ -2593,7 +2593,7 @@ window._gBrowser = {
 
   moveTabsToEnd(contextTab) {
     let tabs = contextTab.multiselected ?
-      gBrowser.selectedTabs :
+      this.selectedTabs :
       [contextTab];
     for (let tab of tabs) {
       if (tab._tPos < this.tabs.length - 1) {