Update appengine-pipelines before files API turndown.

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": "fcd40650bf774bae303a261cc03411a426ac80fc",
 }
 
 deps = {
   "build":
     "https://chromium.googlesource.com/chromium/tools/build.git",
@@ skipping 26 matching lines @@
 
   "infra/appengine/third_party/highlight":
     ("https://chromium.googlesource.com/infra/third_party/highlight.js.git"
      "@fa5bfec38aebd1415a81c9c674d0fde8ee5ef0ba"),
 
   "infra/appengine/third_party/difflibjs":
     ("https://chromium.googlesource.com/external/github.com/qiao/difflib.js.git"
      "@e11553ba3e303e2db206d04c95f8e51c5692ca28"),
 
   "infra/appengine/third_party/pipeline":
-    ("https://chromium.googlesource.com/infra/third_party/"
-     "appengine-pipeline.git"
-     "@e79945156a7a3a48eecb29b792a9b7925631aadf"),
+    ("https://chromium.googlesource.com/external/github.com/"
+     "GoogleCloudPlatform/appengine-pipelines.git"
+     "@58cf59907f67db359fe626ee06b6d3ac448c9e15"),
 
   "infra/appengine/third_party/google-api-python-client":
     ("https://chromium.googlesource.com/external/github.com/google/"
      "google-api-python-client.git"
      "@49d45a6c3318b75e551c3022020f46c78655f365"),
 
   "infra/appengine/third_party/trace-viewer":
     ("https://chromium.googlesource.com/external/"
      "trace-viewer.git"
      '@' + '6c50e784746e6c785b4f5476cf843edecb4fab00'),
@@ skipping 53 matching lines @@
     "pattern": ".",
     "action": [
       "python", "-u", "./infra/bootstrap/get_appengine.py", "--dest=.",
       "--go",
     ],
     # extract in go_appengine/
   },
 ]
 
 recursedeps = ['build']