Bug 1257972 - Follow up, use getActiveExperimentID instead. r=me

getActiveExperimentID() checks for this._experiments being not null, while the other one doesn't. So this one is more resilient to a profile that has not had any previous experiments

MozReview-Commit-ID: 80Spz1syctg

browser/extensions/e10srollout/bootstrap.js

@@ -115,5 +115,5 @@ function optedOut() {
 
 function isThereAnActiveExperiment() {
   let { Experiments } = Cu.import("resource:///modules/experiments/Experiments.jsm", {});
-  return (Experiments.instance().getActiveExperiment() !== null);
+  return (Experiments.instance().getActiveExperimentID() !== null);
 }