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

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 24b7841584f49eeb8f863ee57b033256ad4dd8f2..b288033730502da3d78a84d6b10fe0721f5de083 100644
--- a/DEPS
+++ b/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",
   }
 }