fixup! Regression tests for TB4: Tor Browser's Firefox preference overrides.
Bug #10822: Introduce a pref to disable SSL session tracking

The pref, "security.disable_session_identifiers", disables both SSL3
session IDs and TLS session tracking. To be proposed for
https://bugzilla.mozilla.org/show_bug.cgi?id=967977
Revert "TB5: Disable SSL Session ID tracking."

This reverts commit 378def2b6c0e1c88cd89d52bfe8f0f7a5aa5d22f.
fixup! TB4 (security.disable_session_identifiers pref)