// * An arbitrary identifier key e.g. "cr-extensions-examples".
// * What URL the rule should be invoked with, given by "serveFrom", e.g.
// "extensions/examples".
-// * An object describing where the content originates, either "chromium"
-// or "github".
+// * An object describing where the content originates, either "chromium",
+// "github", or "gcs".
// * "chromium" must provide a "dir" value specifying which chromium directory
// to look in, e.g. "extensions/samples".
// * "github" must provide "owner" and "repo" values specifying the owner of