Remove unused imports in webkitpy with using autoflake.

third_party/WebKit/Tools/Scripts/webkitpy/performance_tests/perftest_unittest.py

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-import StringIO
-import json
-import math
 import unittest
 
 from webkitpy.common.host_mock import MockHost
 from webkitpy.common.system.outputcapture import OutputCapture
 from webkitpy.layout_tests.port.driver import DriverOutput
 from webkitpy.layout_tests.port.test import TestDriver
 from webkitpy.layout_tests.port.test import TestPort
 from webkitpy.performance_tests.perftest import ChromiumStylePerfTest
 from webkitpy.performance_tests.perftest import PerfTest
 from webkitpy.performance_tests.perftest import PerfTestMetric
@@ skipping 195 matching lines @@
 
 class TestPerfTestFactory(unittest.TestCase):
 
     def test_regular_test(self):
         test = PerfTestFactory.create_perf_test(MockPort(), 'some-dir/some-test', '/path/some-dir/some-test')
         self.assertEqual(test.__class__, PerfTest)
 
     def test_inspector_test(self):
         test = PerfTestFactory.create_perf_test(MockPort(), 'inspector/some-test', '/path/inspector/some-test')
         self.assertEqual(test.__class__, ChromiumStylePerfTest)