Bug 1500251 - Add dummy pref with default value true, report via telemetry. r=mythmon

The Webrender Pref Experiment is reporting its pref via telemetry and that is
reporting a different value than what the Normandy experiments telemetry
indicates should be being seen.

In Bug 1499552 I added a pref and telemetry reporting in preparation for running
a dummy Pref Experiment to verify that prefs set are reported as set, but I
realised I missed a case; where there's an existing default pref set,
we're only covering the case where that default pref has value false. We're seeing
a ~25% failure rate in the pref reporting. So we should cover the other three cases
in the dummy Pref Experiment, just in case it's one of these four cases that
is failing.

So here I add another dummy pref with a default value of true, and I report that
via telemetry. I rename the pref I added yesterday to make things consistent.

Differential Revision: https://phabricator.services.mozilla.com/D9156

--HG--
extra : moz-landing-system : lando

modules/libpref/init/all.js

@@ -5941,5 +5941,6 @@ pref("prio.enabled", false);
 #ifdef NIGHTLY_BUILD
 // Bug 1499552; add a dummy pref to verify that collection of preferences
 // via telemetry is working as expected.
-pref("app.normandy.test.with_default", false);
+pref("app.normandy.test.with_true_default", true);
+pref("app.normandy.test.with_false_default", false);
 #endif

toolkit/components/telemetry/app/TelemetryEnvironment.jsm

@@ -183,7 +183,8 @@ const DEFAULT_ENVIRONMENT_PREFS = new Map([
   ["app.support.baseURL", {what: RECORD_PREF_VALUE}],
   ["accessibility.browsewithcaret", {what: RECORD_PREF_VALUE}],
   ["accessibility.force_disabled", {what:  RECORD_PREF_VALUE}],
-  ["app.normandy.test.with_default", {what: RECORD_PREF_VALUE}],
+  ["app.normandy.test.with_true_default", {what: RECORD_PREF_VALUE}],
+  ["app.normandy.test.with_false_default", {what: RECORD_PREF_VALUE}],
   ["app.normandy.test.without_default", {what: RECORD_PREF_VALUE}],
   ["app.shield.optoutstudies.enabled", {what: RECORD_PREF_VALUE}],
   ["app.update.auto", {what: RECORD_PREF_VALUE}],