[Cronet] fix missing python sleep import

Bug: 892666
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;master.tryserver.chromium.android:android_cronet_tester
Change-Id: Ic9a5cedb520d339b458c2208a6dbb6e57be4cc34
Reviewed-on: https://chromium-review.googlesource.com/c/1264898Reviewed-by: Misha Efimov <mef@chromium.org>
Commit-Queue: Paul Jensen <pauljensen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597189}

components/cronet/tools/perf_test_utils.py

@@ -11,6 +11,7 @@ import os
 import posixpath
 import subprocess
 import tempfile
+from time import sleep
 
 REPOSITORY_ROOT = os.path.abspath(os.path.join(
     os.path.dirname(__file__), '..', '..', '..'))