Bug 1834274 - Part 1: Implement nsIRFPService.idl for nsRFPService. r=emilio

To allow remote settings client to set fingerprinting overrides from
Javascript, we need to implement nsIRFPService for nsRFPService.

This patch changes the return format of nsRFPService::GetOrCreate() to
match the singleton implementation of nsIRFPService. As a result of it,
we need to change the first init call in nsContentUtils because we
cannot forward a already_AddRefed<> to mozilla::Unused. It will cause a
memory leak.

Differential Revision: https://phabricator.services.mozilla.com/D182809

dom/base/nsContentUtils.cpp

@@ -825,7 +825,8 @@ nsresult nsContentUtils::Init() {
 
   Element::InitCCCallbacks();
 
-  Unused << nsRFPService::GetOrCreate();
+  RefPtr<nsRFPService> rfpService = nsRFPService::GetOrCreate();
+  MOZ_ASSERT(rfpService);
 
   if (XRE_IsParentProcess()) {
     AsyncPrecreateStringBundles();