Add mock to //.vpython.

Bug: 878777
Change-Id: I7d6727fd36523f37db5f670a46e0957b93cb575d
Reviewed-on: https://chromium-review.googlesource.com/1239166
Commit-Queue: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593320}

.vpython

@@ -175,3 +175,22 @@ wheel: <
   version: "version:4.1"
   match_tag: < platform: "macosx_10_10_intel" >
 >
+
+# Used by:
+#   third_party/catapult
+wheel: <
+  name: "infra/python/wheels/six-py2_py3"
+  version: "version:1.10.0"
+>
+wheel: <
+  name: "infra/python/wheels/pbr-py2_py3"
+  version: "version:3.0.0"
+>
+wheel: <
+  name: "infra/python/wheels/funcsigs-py2_py3"
+  version: "version:1.0.2"
+>
+wheel: <
+  name: "infra/python/wheels/mock-py2_py3"
+  version: "version:2.0.0"
+>