run-perf-tests should generate a json file that summaries the result

run-perf-tests should generate a json file that summaries the result
https://bugs.webkit.org/show_bug.cgi?id=76504

Reviewed by Adam Barth.

Add an ability to generate a json file to run-perf-test in the preparation for perf bots.
New option --outout-json-path specifies the json file's path, and --source-json-path specifies
another json file to be merged into the generated json file.

Also fixed a bug that --build wasn't set by default.

* Scripts/webkitpy/layout_tests/port/test.py:
(TestPort.webkit_base):
* Scripts/webkitpy/performance_tests/perftestsrunner.py:
(PerfTestsRunner.__init__):
(PerfTestsRunner):
(PerfTestsRunner._parse_args):
(PerfTestsRunner.run):
(PerfTestsRunner._generate_json_if_specified):
(PerfTestsRunner._process_chromium_style_test_result):
(PerfTestsRunner._process_parser_test_result):
* Scripts/webkitpy/performance_tests/perftestsrunner_unittest.py:
(MainTest):
(create_runner):
(test_run_test_set):
(test_run_test_set_for_parser_tests):
(test_run_test_set_with_summary_json):
(test_collect_tests):
(test_parse_args):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105256 268f45cc-cd09-0410-ab3c-d52691b4dbfc