MeasurementSetAnalyzer should check triggerable availability before creating confirmi...

MeasurementSetAnalyzer should check triggerable availability before creating confirming A/B tests.
https://bugs.webkit.org/show_bug.cgi?id=187028

Reviewed by Ryosuke Niwa.

If the triggerable is not available, MeasurmentSetAnalyzer should only create analysis task without
confirming A/B tests.

* tools/js/measurement-set-analyzer.js: Added logic to check triggerable availability.
(MeasurementSetAnalyzer.prototype.async._analyzeMeasurementSet):
(MeasurementSetAnalyzer):
* unit-tests/measurement-set-analyzer-tests.js: Updated unit tests and added a new unit test for this change.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@233326 268f45cc-cd09-0410-ab3c-d52691b4dbfc