GetValue is going to be removed in a subsequent patch. It is no longer needed,
because it can be replaced by functions already provided by nsBaseHashtable,
in particular Lookup and Contains.
Also, its name was confusing, since it specifically returns a pointer that
allows and is intended for modifying the entry within the hashtable, rather
than returning by-value. According to the naming rules to be set on
nsBaseHashtable, it would also needed to be renamed to "Lookup*. Removing
its uses saves this effort.
Differential Revision: https://phabricator.services.mozilla.com/D105476
8ecfb38a41