[ChromeDriver] Retry server init in WPT test

Due to occasional ChromeDriver startup timeout on Linux Tests (dbg)
builder, modify run_webdriver_tests.py to retry starting ChromeDriver.

Bug: 931586
Change-Id: I6eb4c0e01f857bff88b26647a9aec67159b898be
Reviewed-on: https://chromium-review.googlesource.com/c/1478332
Commit-Queue: John Chen <johnchen@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633745}

chrome/test/chromedriver/test/run_webdriver_tests.py

@@ -159,7 +159,12 @@ if __name__ == '__main__':
     parser.error('Path given by --chromedriver is invalid.\n' +
                  'Please run "%s --help" for help' % __file__)
 
-  chromedriver_server = server.Server(options.chromedriver, options.log_path)
+  # Due to occasional timeout in starting ChromeDriver, retry once when needed.
+  try:
+    chromedriver_server = server.Server(options.chromedriver, options.log_path)
+  except RuntimeError as e:
+    _log.warn('Error starting ChromeDriver, retrying...')
+    chromedriver_server = server.Server(options.chromedriver, options.log_path)
 
   if not chromedriver_server.IsRunning():
     _log.error('ChromeDriver is not running.')