Roll virtualenv 12.0->15.0.2

DEPS

 vars = {
   # npm_modules.git is special: we can't check it out on Windows because paths
   # there are too long for Windows. Instead we use 'deps_os' gclient feature to
   # checkout it out only on Linux and Mac.
   "npm_modules_revision": "029f11ae7e3189fd57f7764b4bb43d0900e16c92",
 }
 
 deps = {
   "build":
     "https://chromium.googlesource.com/chromium/tools/build.git",
@@ skipping 77 matching lines @@
   "infra/appengine/third_party/dateutil":
     ("https://chromium.googlesource.com/external/code.launchpad.net/dateutil/"
      "@8c6026ba09716a4e164f5420120bfe2ebb2d9d82"),
 
   ## For ease of development. These are pulled in as wheels for run.py/test.py
   "expect_tests":
     "https://chromium.googlesource.com/infra/testing/expect_tests.git",
   "testing_support":
     "https://chromium.googlesource.com/infra/testing/testing_support.git",
 
-  # v12.0
-  "infra/bootstrap/virtualenv":
-    ("https://chromium.googlesource.com/external/github.com/pypa/virtualenv.git"
-     "@4243b272823228dde5d18a7400c404ce52fb4cea"),
-
   "infra/appengine/third_party/src/github.com/golang/oauth2":
   ("https://chromium.googlesource.com/external/github.com/golang/oauth2.git"
    "@cb029f4c1f58850787981eefaf9d9bf547c1a722"),
 }
 
 
 deps_os = {
   "unix": {
     "infra/appengine/third_party/npm_modules":
       ("https://chromium.googlesource.com/infra/third_party/npm_modules.git@" +
@@ skipping 37 matching lines @@
   },
   {
     "pattern": ".",
     "action": [
       "python", "-u", "./infra/bootstrap/install_cipd_packages.py", "-v",
     ],
   },
 ]
 
 recursedeps = ['build']