Add infrastructure to download golang and build AppRTC collider.

DEPS

Index: DEPS
===================================================================
--- DEPS	(revision 293292)
+++ DEPS	(working copy)
@@ -40,10 +40,16 @@
   {
     "pattern": ".",
     "action" : ["python",
-                "webrtc.DEPS/download_appengine_sdk.py",
+                "webrtc.DEPS/download_appengine_and_mercurial.py",
                 "webrtc.DEPS"],
   },
   {
+    "pattern": ".",
+    "action" : ["python",
+                "webrtc.DEPS/download_golang.py",
+                "webrtc.DEPS"],
+  },
+  {
     # "Build" AppRTC, i.e. move it to the out/ dir where the browser test
     # can find it. This is only done on runhooks.
     "pattern": ".",
@@ -51,6 +57,18 @@
                 "webrtc.DEPS/copy_apprtc.py"],
   },
   {
+    # Build Mercurial which is needed by the golang toolchain.
+    "pattern": ".",
+    "action" : ["python",
+                "webrtc.DEPS/build_mercurial_local.py"],
+  },
+  {
+    # Build AppRTC Collider using the golang toolchain.
+    "pattern": ".",
+    "action" : ["python",
+                "webrtc.DEPS/build_apprtc_collider.py"],
+  },
+  {
     # Download test resources, i.e. video and audio files from Google Storage.
     # These resources are used by the Android bots.
     "pattern": "\\.sha1",