Add infrastructure to download golang and build AppRTC collider.

DEPS

 # DEPS file that pulls down dependencies that are needed to build and
 # execute WebRTC specific tests from a Chromium checkout.
 
 vars = {
   'git_url':
      'https://chromium.googlesource.com'
 }
 
 deps = {
   'webrtc-samples':
@@ skipping 22 matching lines @@
       Var('git_url') + '/external/webrtc/trunk/third_party/gflags.git',
     'src/third_party/gflags/src':
       Var('git_url') + '/external/gflags/src.git@e7390f9185c75f8d902c05ed7d20bb94eb914d0c',
     },
 }
 
 hooks = [
   {
     "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": ".",
     "action" : ["python",
                 "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",
     "action": ["download_from_google_storage",
                "--directory",
                "--recursive",
                "--num_threads=10",
                "--bucket", "chromium-webrtc-resources",
                "src/resources"],
   },
@@ skipping 20 matching lines @@
   },
   {
     # Download firefox for the Firefox AppRTC test.
     "pattern": ".",
     "action" : ["python",
                 "webrtc.DEPS/download_firefox_nightly.py",
                 "-t",
                 "firefox-nightly"],
   },
 ]