Add mirror for downloading Bazel rules.

Bazel rules are currently downloaded from GitHub which
make the whole build very fragile and subject to failures
when GitHub is temporarily unavailable.

Add another mirror using GerritForge's CI server so that
the build can have another source of the Bazel rules
should GitHub be unreachable.

Bug: Issue 12542
Change-Id: Iefa4a299000a85e66f6c9704963ce368a87adef0
diff --git a/WORKSPACE b/WORKSPACE
index 20f02ee..bfbb34b 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -29,7 +29,10 @@
     name = "io_bazel_rules_closure",
     sha256 = "b9c2bc6ba377aa497eb7c31681d34404febf9d4e3c9c7d98ce0d78238a0af20f",
     strip_prefix = "rules_closure-0.31",
-    urls = ["https://github.com/davido/rules_closure/archive/V0.31.tar.gz"],
+    urls = [
+        "https://github.com/davido/rules_closure/archive/V0.31.tar.gz",
+        "https://gerrit-ci.gerritforge.com/lib/V0.31.tar.gz",
+    ],
 )
 
 # File is specific to Polymer and copied from the Closure Github -- should be