perf-o-matic should incrementally update JSON responses

perf-o-matic should incrementally update JSON responses
https://bugs.webkit.org/show_bug.cgi?id=79898

Reviewed by Eric Seidel.

Now that Runs object store test_runs and averages separately,
we can update JSON data incrementally without degrading values.

Also fixed the order of arguments passed to DashboardImage.needs_update
in schedule_runs_update. This bug had caused all chart images to be
updated on every new report.

* Websites/webkit-perf.appspot.com/controller.py:
(schedule_runs_update):
* Websites/webkit-perf.appspot.com/models.py:
(Runs.update_or_insert):
(Runs.update_incrementally):
(Runs):
(Runs.get_by_objects):
* Websites/webkit-perf.appspot.com/models_unittest.py:
(RunsTest._create_results):
(RunsTest.test_generate_runs):
(RunsTest.test_update_or_insert):
(RunsTest.test_update_incrementally):
(RunsTest.test_to_json_with_results):
* Websites/webkit-perf.appspot.com/report_process_handler.py:
(ReportProcessHandler.post):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@110210 268f45cc-cd09-0410-ab3c-d52691b4dbfc