Kill any lingering spawning test server on Android when handling /start

BUG=665686

Review-Url: https://codereview.chromium.org/2508873003
Cr-Original-Commit-Position: refs/heads/master@{#432788}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 75b27526ecaf8fe51b180639e2c9a46b87613dd9

android/pylib/chrome_test_server_spawner.py

@@ -321,7 +321,14 @@ class SpawningServerRequestHandler(BaseHTTPServer.BaseHTTPRequestHandler):
     logging.info(content_length)
     test_server_argument_json = self.rfile.read(content_length)
     logging.info(test_server_argument_json)
-    assert not self.server.test_server_instance
+    # There should only be one test server instance at a time. However it may
+    # be possible that a previous instance was not cleaned up properly
+    # (crbug.com/665686)
+    if self.server.test_server_instance:
+      port = self.server.test_server_instance.host_port
+      logging.info('Killing lingering test server instance on port: %d', port)
+      self.server.test_server_instance.Stop()
+      self.server.test_server_instance = None
     ready_event = threading.Event()
     self.server.test_server_instance = TestServerThread(
         ready_event,