Now, we have `nsFocusManager::GetFocusedElementStatic()` which returns focused
element if the `nsFocusManager` instance is available. Therefore, if
`nsFocusManager::GetFocusedElement()` users do not use other methods of
`nsFocusManager`, they can use `nsFocusManager::GetFocusedElementStatic()` and
make themselves simpler.
Note that some callers return early if `nsFocusManager` is not available, but
they do not return error and `nsFocusManager` instance is available in most
time of the life time of the process. Therefore, we can simply stop using the
early return.
Differential Revision: https://phabricator.services.mozilla.com/D217527
44aae6e962