AU: stable channel

This adds a stable-channel track to the updater.

BUG=chromium-os:14071
TEST=unittests

Change-Id: I6ed83594134c06e160537638974473c119bd8874

Review URL: http://codereview.chromium.org/6823087
diff --git a/omaha_request_params.cc b/omaha_request_params.cc
index ed80060..f4f8fb2 100644
--- a/omaha_request_params.cc
+++ b/omaha_request_params.cc
@@ -185,6 +185,7 @@
 bool OmahaRequestDeviceParams::IsValidTrack(const std::string& track) const {
   static const char* kValidTracks[] = {
     "canary-channel",
+    "stable-channel",
     "beta-channel",
     "dev-channel",
   };