Don't wrap tab info in Tab if Tab is not defined

chrome/common.js

Index: chrome/common.js
===================================================================
--- a/chrome/common.js
+++ b/chrome/common.js
@@ -70,7 +70,7 @@
     _wrapListener: function(listener) {
       return function(message, sender, sendResponse)
       {
-        if (sender.tab && sender.tab.id >= 0)
+        if ("Tab" in window && sender.tab && sender.tab.id >= 0)

[Wladimir Palant, 2014/01/16 08:01:10]

So we have content scripts receive messages where sender.tab is set? That's
weird to say the least. Well, fine with me then...

[Thomas Greiner, 2014/01/16 10:15:30]

Yes, that is the case and it seems like it's intentional:

"The onMessage event is fired in each extension page of the extension."
http://developer.chrome.com/extensions/runtime.html#method-sendMessage

"The tabs.Tab which opened the connection, if any. This property will only be
present when the connection was opened from a tab (including content scripts),
and only if the receiver is an extension, not an app."
http://developer.chrome.com/extensions/runtime.html#type-MessageSender

According to this definition the message (including the tab property) is not
only sent to the background page but also to firstRun.html from the content
script running within the iFrame showing share.adblockplus.org

[Wladimir Palant, 2014/01/16 10:44:49]

Ok, this actually makes sense then.

           sender.tab = new Tab(sender.tab);
         return listener(message, sender, sendResponse);
       };