Enable SVG page loading performance tests

Enable SVG page loading performance tests
https://bugs.webkit.org/show_bug.cgi?id=83861

Reviewed by Adam Barth.

PerformanceTests:

Unskip PageLoad directory and skip tests that load too fast (less than 100ms) and are unstable,
and the ones that take too long to load (more than 20s).

* Skipped:

Tools:

Add support for page loading tests. These are tests without any test running harnesses, and we use
DriverOutput's test_timeout to measure the loading time.

Extracted PerfTest.run and PerfTest.run_failed from PerfTestRunner._run_single_test.

* Scripts/webkitpy/performance_tests/perftest.py:
(PerfTest.run):
(PerfTest):
(PerfTest.run_failed):
(PerfTest.parse_output):
(PerfTest.output_statistics):
(ChromiumStylePerfTest.parse_output):
(PageLoadingPerfTest): Added.
(PageLoadingPerfTest.__init__):
(PageLoadingPerfTest.run): Run the driver 20 times to get the statistics for the loading time.
* Scripts/webkitpy/performance_tests/perftest_unittest.py: Added.
(MockPrinter):
(MockPrinter.__init__):
(MockPrinter.write):
(MainTest):
(MainTest.test_parse_output):
(MainTest.test_parse_output_with_failing_line):
(TestPageLoadingPerfTest):
(TestPageLoadingPerfTest.assertWritten):
(TestPageLoadingPerfTest.MockDriver):
(TestPageLoadingPerfTest.MockDriver.__init__):
(TestPageLoadingPerfTest.MockDriver.run_test):
(TestPageLoadingPerfTest.test_run):
(TestPageLoadingPerfTest.test_run_with_bad_output):
* Scripts/webkitpy/performance_tests/perftestsrunner.py:
(PerfTestsRunner):
(PerfTestsRunner._collect_tests._is_test_file): Now recognizes svg tests.
(PerfTestsRunner._collect_tests):
(PerfTestsRunner._run_tests_set):
(PerfTestsRunner._run_single_test):
* Scripts/webkitpy/performance_tests/perftestsrunner_unittest.py:
(test_collect_tests_with_page_load_svg):
(test_collect_tests_with_page_load_svg.add_file):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@114149 268f45cc-cd09-0410-ab3c-d52691b4dbfc