Bug 1388066 - Fix message sender.tab when tabId is 0 on Firefox for Android. r=mixedpuppy,zombie

MozReview-Commit-ID: DPu6EZVg1nN

--HG--
extra : rebase_source : 95578d235fd065ae84ac5e2d3a307321854e1d14

mobile/android/components/extensions/ext-android.js

@@ -6,7 +6,7 @@ XPCOMUtils.defineLazyModuleGetter(global, "EventEmitter",
 // This function is pretty tightly tied to Extension.jsm.
 // Its job is to fill in the |tab| property of the sender.
 const getSender = (extension, target, sender) => {
-  let tabId;
+  let tabId = -1;
   if ("tabId" in sender) {
     // The message came from a privileged extension page running in a tab. In
     // that case, it should include a tabId property (which is filled in by the
@@ -17,7 +17,7 @@ const getSender = (extension, target, sender) => {
     tabId = tabTracker.getBrowserData(target).tabId;
   }
 
-  if (tabId) {
+  if (tabId != null && tabId >= 0) {
     let tab = extension.tabManager.get(tabId, null);
     if (tab) {
       sender.tab = tab.convert();