Allow chromium url to be overwritten as a custom variable in DEPS.

Using this work around, buildbot can set the chromium_url variable to the golo mirror.

BUG=372738
LOG=n
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/281053003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21296 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

DEPS

@@ -2,22 +2,26 @@
 # directory and assume that the root of the checkout is in ./v8/, so
 # all paths in here must match this assumption.
 
+vars = {
+  "chromium_url": "https://src.chromium.org",
+}
+
 deps = {
   # Remember to keep the revision in sync with the Makefile.
   "v8/build/gyp":
     "http://gyp.googlecode.com/svn/trunk@1831",
 
   "v8/third_party/icu":
-    "https://src.chromium.org/svn/trunk/deps/third_party/icu46@258359",
+    Var("chromium_url") + "/svn/trunk/deps/third_party/icu46@258359",
 }
 
 deps_os = {
   "win": {
     "v8/third_party/cygwin":
-      "http://src.chromium.org/svn/trunk/deps/third_party/cygwin@66844",
+      Var("chromium_url") + "/svn/trunk/deps/third_party/cygwin@66844",
 
     "v8/third_party/python_26":
-      "http://src.chromium.org/svn/trunk/tools/third_party/python_26@89111",
+      Var("chromium_url") + "/svn/trunk/tools/third_party/python_26@89111",
   }
 }