Implement Windows GPO support for "dlpref" in component updater.

components/update_client/utils.h

Index: components/update_client/utils.h
diff --git a/components/update_client/utils.h b/components/update_client/utils.h
index 83d922e77efc9e6f4e3fff2f1d0ca3b9b0e9fdfe..b1d7e36ce6a99e41df36eaf86ba96090b2a20462 100644
--- a/components/update_client/utils.h
+++ b/components/update_client/utils.h
@@ -44,6 +44,8 @@ struct CrxUpdateItem;
 
 // Builds a protocol request string by creating the outer envelope for
 // the request and including the request body specified as a parameter.
+// If present, the |download_preference| specifies a group policy that
+// affects the list of download URLs returned in the update response.
 // If specified, |additional_attributes| are appended as attributes of the
 // request element. The additional attributes have to be well-formed for
 // insertion in the request element.
@@ -51,6 +53,7 @@ std::string BuildProtocolRequest(const std::string& browser_version,
                                  const std::string& channel,
                                  const std::string& lang,
                                  const std::string& os_long_name,
+                                 const std::string& download_preference,
                                  const std::string& request_body,
                                  const std::string& additional_attributes);