add -f option to force a client update

Review URL: http://codereview.chromium.org/503088
diff --git a/autoupdate.py b/autoupdate.py
index f2f3183..2f0fe95 100644
--- a/autoupdate.py
+++ b/autoupdate.py
@@ -95,7 +95,7 @@
     client_version = query.attributes['version'].value
     latest_image_path = self.GetLatestImagePath();
     latest_version = self.GetLatestVersion(latest_image_path);
-    if not self.CanUpdate(client_version, latest_version):
+    if client_version != "ForcedUpdate" and not self.CanUpdate(client_version, latest_version):
       web.debug("no update")
       return self.GetNoUpdatePayload()