Roll src/tools/swarming_client/ df99a00d9..9cdd76171 (4 commits).

https://chromium.googlesource.com/external/swarming.client.git/+log/df99a00d96fa..9cdd76171e51

$ git log df99a00d9..9cdd76171 --date=short --no-merges --format='%ad %ae %s'
2015-12-02 maruel Fix undeleteable directory.
2015-12-01 maruel Make collect more resistant to transient error.
2015-12-01 maruel Fix FormatError() encoding issue on non en-US locale.
2015-11-24 maruel Improve tools/count.py output.

Mostly for the most recent commit.

TBR=vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1493223002

Cr-Commit-Position: refs/heads/master@{#362975}

DEPS

@@ -47,7 +47,7 @@ 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': 'df99a00d96fae932bae824dccba13156bf7eddd0',
+  'swarming_revision': '9cdd76171e517a430a72dcd7d66ade67e109aa00',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling ANGLE
   # and whatever else without interference from each other.