Roll src/tools/swarming_client to a56c2b39ca2.

https://chromium.googlesource.com/external/swarming.client.git/+log/af6b06ca68..a56c2b39ca2

This will eventually allow to trigger Swarming jobs from LUCI chromium builds
using LUCI service accounts.

BUG=730878
R=iannucci@chromium.org

Review-Url: https://codereview.chromium.org/2957063004
Cr-Commit-Position: refs/heads/master@{#483134}

DEPS

@@ -48,7 +48,7 @@ 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': 'af6b06ca68ba7a618024f28856418296a9acf375',
+  'swarming_revision': 'a56c2b39ca23bdf41458421a7f825ddbf3f43f28',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling ANGLE
   # and whatever else without interference from each other.