Roll src/tools/swarming_client/ b6e9e23e4..157bec8a2 (4 commits)

https://chromium.googlesource.com/infra/luci/client-py.git/+log/b6e9e23e4e79..157bec8a25cc

$ git log b6e9e23e4..157bec8a2 --date=short --no-merges --format='%ad %ae %s'
2018-11-26 bradhall Make sure to setup_googletest on all slices
2018-11-26 maruel [client] Add warning about variable flags
2018-11-21 maruel [client] Stop leaking dir 'cache' when running isolateserver_test.py
2018-11-20 maruel [client] internal refactoring adding ServerRef

Created with:
  roll-dep src/tools/swarming_client

R=bradhall@google.com

Bug: 871453, 908551
Change-Id: Ia0d51cc0584d6df90b71722fbbc9a17bbbceb563
Reviewed-on: https://chromium-review.googlesource.com/c/1351743Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611194}

DEPS

@@ -120,7 +120,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling swarming_client
   # and whatever else without interference from each other.
-  'swarming_revision': 'b6e9e23e4e79249bd4f95735205ffb7c3f9f0912',
+  'swarming_revision': '157bec8a25cc4ebd6a16052510d08b05b6102aad',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling ANGLE
   # and whatever else without interference from each other.