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

This caused errors in the first-run page and was reported to us here https://adblockplus.org/forum/viewtopic.php?f=10&t=20365#p90014.

[Wladimir Palant, 2014-01-15 15:44:08]

This sounds like curing symptoms without understanding the problem. What's
causing it, are we receiving a message in a content script that was already
unloaded?

[Thomas Greiner, 2014-01-15 17:52:14]

On 2014/01/15 15:44:08, Wladimir Palant wrote:
> This sounds like curing symptoms without understanding the problem. What's
> causing it, are we receiving a message in a content script that was already
> unloaded?

Due to Tab being declared in */background.js, and */common.js being also used in
content scripts, this missing check appears to be a simple oversight on our end
on the time of writing this code.

This function is used in either environment so the solution is either to
 (a) check in */common.js for Tab to be available,
 (b) create a minimal implementation of Tab in */content.js or
 (c) implement as much of Tab in */commons.js as possible and extend it in
*/background.js (e.g. with ext.browserAction).

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

LGTM

http://codereview.adblockplus.org/6184947589054464/diff/5629499534213120/chro...
File chrome/common.js (right):

http://codereview.adblockplus.org/6184947589054464/diff/5629499534213120/chro...
chrome/common.js:73: if ("Tab" in window && sender.tab && sender.tab.id >= 0)
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]

http://codereview.adblockplus.org/6184947589054464/diff/5629499534213120/chro...
File chrome/common.js (right):

http://codereview.adblockplus.org/6184947589054464/diff/5629499534213120/chro...
chrome/common.js:73: if ("Tab" in window && sender.tab && sender.tab.id >= 0)
On 2014/01/16 08:01:10, Wladimir Palant wrote:
> So we have content scripts receive messages where sender.tab is set? That's
> weird to say the least. Well, fine with me then...

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

[Thomas Greiner, 2014-01-16 10:22:46]

"[...] but also to firstRun.html from the content
script running within the iFrame showing share.adblockplus.org" <- This seems to
apply to all frames within firstRun.html.

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

http://codereview.adblockplus.org/6184947589054464/diff/5629499534213120/chro...
File chrome/common.js (right):

http://codereview.adblockplus.org/6184947589054464/diff/5629499534213120/chro...
chrome/common.js:73: if ("Tab" in window && sender.tab && sender.tab.id >= 0)
On 2014/01/16 10:15:30, Thomas Greiner wrote:
> 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 http://share.adblockplus.org

Ok, this actually makes sense then.