Roll tools/swarming_client/ to a57d7db4026bca8e0fab9aadd4953503e8d74e74.

$ git log f9e401bab..a57d7db40 --date=short --format='%ad %ae %s' | sed 's/@chromium\.org//'
2014-10-15 maruel Add --cache to isolateserver.py download.
2014-10-15 maruel Preparation work to enable isolateserver.py download to use a local cache.

TBR=vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/664493002

Cr-Commit-Position: refs/heads/master@{#300023}

DEPS

@@ -56,7 +56,7 @@ 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': 'f9e401bab51a7eb0e92ae70aab73c2bcab6d34d7',
+  'swarming_revision': 'a57d7db4026bca8e0fab9aadd4953503e8d74e74',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling ANGLE
   # and whatever else without interference from each other.