The browser.contentPrincpal will report a null prinicpal instead of the actual
content principal if the tab is not loaded. So the SessionStore will collect a
wrong principal for the 'iconLoadingPrincipal', and it will use this wrong
principal to load favicon when session restoring.
To fix this problem, this patch makes the TabState.jsm to collect
'iconLoadingPrincipal' from browser.mIconLoadingPrincipal which will be the
correct principal for loading favicon.
MozReview-Commit-ID: AYUbHFKaG8v