Add coverage to src's root vpython spec.

Looks like we already have the wheel:
https://codesearch.chromium.org/chromium/infra/recipes-py/.vpython?rcl=0f093b369402a86363de15a3fae0c98eab4c8e00&l=5

Bug: 792130
Change-Id: I451bc2abe4d801248101536a998f77bba466e6c4
Reviewed-on: https://chromium-review.googlesource.com/812492Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Benjamin Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522466}

.vpython

@@ -98,3 +98,10 @@ wheel: <
   name: "infra/python/wheels/boto-py2_py3"
   version: "version:2.48.0"
 >
+
+# Used by:
+#   testing/buildbot/generate_buildbot_json_coveragetest.py
+wheel: <
+  name: "infra/python/wheels/coverage/${vpython_platform}"
+  version: "version:4.3.4"
+>