Switch to mirror for third-party/markdown checkout.

BUG=
R=sgjesse@google.com

Review URL: https://codereview.chromium.org//1300793002 .
diff --git a/DEPS b/DEPS
index ffbcf63..c1077da9 100644
--- a/DEPS
+++ b/DEPS
@@ -222,8 +222,7 @@
   Var("dart_root") + "/third_party/pkg/logging":
       (Var("github_mirror") % "logging") + Var("logging_rev"),
   Var("dart_root") + "/third_party/pkg/markdown":
-      "https://github.com/dart-lang/" +
-      "markdown.git" + Var("markdown_rev"),
+      (Var("github_mirror") % "markdown") + Var("markdown_rev"),
   Var("dart_root") + "/third_party/pkg/matcher":
       (Var("github_mirror") % "matcher") + Var("matcher_tag"),
   Var("dart_root") + "/third_party/pkg/metatest":