Bug 1090430 - Make sure a browser is still remote before showing the busy spinner. r=mconley

A browsers remoteness could change between the time we checked it and
when we make the call to show the spinner. We now perform another
remoteness check just before calling _showBusySpinnerRemoteBrowser().

--HG--
extra : rebase_source : b90cbc5c55497d269a97078013027ba8d9d0196a