Check whether GetChromeOuterWindowID() returns the correct value

[llewelld]

A new BrowserChildHelper::GetChromeOuterWindowID() method was added in commit 44af543e4a360eba3f0198e273c5a39f7bb0b8a7 in order to fulfil the parent interface.

This currently returns the outerWindowID, but perhaps it should be returning something else?

The task is to find out what it should be returning and fix it if necessary.