Bug 16342819 - Use nsTHashMap instead of nsDataHashtable. r=xpcom-reviewers,necko-reviewers,jgilbert,nika,valentin

Note that this patch only transforms the use of the nsDataHashtable type alias
to a directly equivalent use of nsTHashMap. It does not change the specification
of the hash key type to make use of the key class deduction that nsTHashMap
allows for in some cases. That can be done in a separate step, but requires more
attention.

Differential Revision: https://phabricator.services.mozilla.com/D106008