nsILoadGroupChild provides a hierarchy of load groups so that the
root load group can be used to conceptually tie a series of loading
operations into a logical whole while still leaving them separate
for the purposes of cancellation and status events.
The parent of this load group. It is stored with
a nsIWeakReference/nsWeakPtr so there is no requirement for the
parentLoadGroup to out live the child, nor will the child keep a
reference count on the parent.
The nsILoadGroup associated with this nsILoadGroupChild
The rootLoadGroup is the recursive parent of this
load group where parent is defined as parentlLoadGroup if set
or childLoadGroup.loadGroup as a backup. (i.e. parentLoadGroup takes
precedence.) The nsILoadGroup child is the root if neither parent
nor loadgroup attribute is specified.