Roll src/tools/swarming_client/ 6fd3c7b6e..4bd9152f8 (15 commits)

https://chromium.googlesource.com/infra/luci/client-py.git/+log/6fd3c7b6eb7c..4bd9152f8a97

$ git log 6fd3c7b6e..4bd9152f8 --date=short --no-merges --format='%ad %ae %s'
2017-12-05 maruel client/tools: add reproduce.py
2017-12-04 maruel client: isolated unicode fix for Windows
2017-12-04 maruel client: isolated unicode fix on Ubuntu
2017-12-04 maruel tests: more unicode
2017-12-01 maruel client: Fix emoji handling at various places
2017-12-01 maruel example: put tons of emojis in the examples
2017-11-29 maruel subprocess42: be more tolerant to unicode.
2017-11-29 maruel Fix regression in fc3c34e59202ee and bfab799cddab1
2017-11-28 maruel swarming_bot: migrate env mutation from task_runner to run_isolated.
2017-11-23 huangwe Allow None response when the item size is 0 in gRPC isolate push
2017-11-22 iannucci [swarming] Add env_prefixes TaskProperty.
2017-11-20 huangwe Raise gRPC error when gRPC calls for pushing blobs return None
2017-11-13 maruel client: fix missing lock in DiskCache.cleanup()
2017-11-08 vadimsh Refactor httpserver_mock in attempt to get rid of deadlocks.
2017-11-08 vadimsh Add minimal support for Cavium CPUs.

Created with:
  roll-dep src/tools/swarming_client

TBR=iannucci@chromium.org

Bug: 
Change-Id: Id39cf767dc415f16c3c08e7bf8340b58a56834f5
Reviewed-on: https://chromium-review.googlesource.com/811205Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522237}

DEPS

@@ -82,7 +82,7 @@ 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': '6fd3c7b6eb7c60f89e83f8ab1f93c133488f984e',
+  'swarming_revision': '4bd9152f8a975d57c972c071dfb4ddf668e02200',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling ANGLE
   # and whatever else without interference from each other.