[build] Fix swarming_client DEPS url

The repo url changed recently in Chromium, which breaks the V8 DEPS roller.

This uses the same url like Chromium again.

TBR=yangguo@chromium.org

Change-Id: Iffb9daa8380d111e77ebcef1da58082e346c1e4a
Reviewed-on: https://chromium-review.googlesource.com/650348Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47821}

DEPS

@@ -24,7 +24,7 @@ deps = {
   "v8/third_party/markupsafe":
     Var("chromium_url") + "/chromium/src/third_party/markupsafe.git" + "@" + "8f45f5cfa0009d2a70589bcda0349b8cb2b72783",
   "v8/tools/swarming_client":
-    Var('chromium_url') + '/external/swarming.client.git' + '@' + "42721e128da760b345ab60d7cf34e300269112d7",
+    Var('chromium_url') + '/infra/luci/client-py.git' + '@' + "42721e128da760b345ab60d7cf34e300269112d7",
   "v8/testing/gtest":
     Var("chromium_url") + "/external/github.com/google/googletest.git" + "@" + "6f8a66431cb592dad629028a50b3dd418a408c87",
   "v8/testing/gmock":