Perf dashboard should be able to trigger A/B testing jobs for iOS

Perf dashboard should be able to trigger A/B testing jobs for iOS
https://bugs.webkit.org/show_bug.cgi?id=143398

Reviewed by Chris Dumez.

Fix various bugs in the perf dashboard so that it can schedule A/B testing jobs for iOS.

Also generalized sync-with-buildbot.py slightly to meet the requirements of iOS builders.

* public/api/triggerables.php:
(main): Avoid spitting a warning when $id_to_triggerable doesn't contain the triggerable.
* public/v2/analysis.js:
(App.AnalysisTask.triggerable): Log an error when failed to fetch triggerables for debugging purposes.
* public/v2/app.js:
(App.AnalysisTaskController.updateRootConfigurations): Show 'None' when a revision is missing from
some of the data points. This will happen when we modify the list of projects we build for iOS.
(App.AnalysisTaskController.actions.createTestGroup): Gracefully fail by showing alerts when an user
attempts to create an invalid test group; when there is already another test group of the same or when
only either configuration specifies the revision for some repository.
(App.AnalysisTaskController._updateRootsBySelectedPoints): Fixed a typo: sets[i] -> set.
* public/v2/index.html: Don't show the form to create a new test group if it's not available.
* tools/sync-with-buildbot.py:
(find_request_updates):
(schedule_request): iOS builders take a JSON that contains the list of roots. Generate this JSON when
a dictionary of the form {rootsExcluding: ["WebKit"]} is specified. Also replaced the way we refer to
a revision from $-based text replacements to an explicit dictionary of the form {root: "WebKit"}.
(request_id_from_build): Don't hard code the parameter name here. Retrieve the name from the config.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@182438 268f45cc-cd09-0410-ab3c-d52691b4dbfc