Fixed various issues in older Chrome versions

* There was an intermediate messaging API where messages were sent via  chrome.extension.sendMessage/chrome.tabs.sendMessage and received with chrome.extension.onMessage
* Accessing (not calling) chrome.extension.getBackgroundPage used to cause an exception
* Messages received by content scripts got a fake sender tab with ID -1
* Pop-up should not assume always getting a response to its messages

[Sebastian Noack, 2013-12-19 10:38:58]

http://codereview.adblockplus.org/5867629902299136/diff/5629499534213120/chro...
File chrome/common.js (right):

http://codereview.adblockplus.org/5867629902299136/diff/5629499534213120/chro...
chrome/common.js:97: try
I would prefer to wrap chrome.extension.getBackgroundPage() in a function
instead of catching the exception here. So the caller gets the original
exception.

  ext = {
    backgroundPage:
    {
      getWindow: function()
      {
        return chrome.extension.getBackgroundPage();
      }
    },
    ...
  }

[Wladimir Palant, 2013-12-19 10:47:17]

http://codereview.adblockplus.org/5867629902299136/diff/5629499534213120/chro...
File chrome/common.js (right):

http://codereview.adblockplus.org/5867629902299136/diff/5629499534213120/chro...
chrome/common.js:97: try
True, that's a better solution - done.

[Sebastian Noack, 2013-12-19 10:52:09]

LGTM