Don't update the save card bubble icon once the browser isn't available.
If a tab showing the save credit card buble is closed while the bubble
is still visible, the bubble will attempt to update the icon in the
location bar. Because the browser object is no longer available, this
causes a crash.
Note that this doesn't happen in the common case where the user
closes the tab by clicking on the tab close icon (x) because that action
closes the bubble first. But if the user closes the tab via a keyboard