[ChromeDriver] Fix ExistingUserDataDirTest

Add a missing call to base class tearDown in ExistingUserDataDirTest, to
fix breakage in waterfall.

Bug: chromedriver:2446
Change-Id: Ibf0356c53edb92d729aa3d5b8de01a1471fc8bdf
Reviewed-on: https://chromium-review.googlesource.com/1090040Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565120}

chrome/test/chromedriver/test/run_py_tests.py

@@ -2623,6 +2623,7 @@ class ExistingUserDataDirTest(ChromeDriverBaseTest):
     self.user_data_dir = tempfile.mkdtemp()
 
   def tearDown(self):
+    super(ExistingUserDataDirTest, self).tearDown()
     shutil.rmtree(self.user_data_dir, ignore_errors=True)
 
   def testStartUpWithExistingDevToolsPortFile(self):