Bug 21268: e10s compatibility for New Identity.

Ignore "not available" errors from appCacheStorage.asyncEvictStorage()
since these can occur if the cache has not been used yet.

Change torbutton_close_tabs_on_new_identity() to use browser.removeTab()
instead of browser.contentWindow.close() since we cannot directly access
the contentWindow when multi-process mode is enabled.

Fix indentation and modernize the torbutton_close_tabs_on_new_identity()
code.