Bug 1450170 - Add Nightly-only observer notification that fires anytime a tab switch spinner is displayed. r=dthayer

MozReview-Commit-ID: 3S34IVb9BEf

--HG--
extra : rebase_source : 084a98ece1ffa0d32109811d700f80e3e2b1eaf0

browser/modules/AsyncTabSwitcher.jsm

@@ -1033,6 +1033,9 @@ class AsyncTabSwitcher {
     Services.telemetry
       .getHistogramById("FX_TAB_SWITCH_SPINNER_VISIBLE_TRIGGER")
       .add(this._loadTimerClearedBy);
+    if (AppConstants.NIGHTLY_BUILD) {
+      Services.obs.notifyObservers(null, "tabswitch-spinner");
+    }
   }
 
   spinnerHidden() {