Add an about:flag to support alternative data saver features

components/data_reduction_proxy/core/common/data_reduction_proxy_switches.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/data_reduction_proxy/core/common/data_reduction_proxy_switches.h"
 
 namespace data_reduction_proxy {
 namespace switches {
 
 // The origin of the data reduction proxy.
 const char kDataReductionProxy[]         = "spdy-proxy-auth-origin";
 
 // The URL from which to retrieve the Data Reduction Proxy configuration.
 const char kDataReductionProxyConfigURL[] = "data-reduction-proxy-config-url";
 
 // The name of a Data Reduction Proxy experiment to run. These experiments are
 // defined by the proxy server. Use --force-fieldtrials for Data Reduction
 // Proxy field trials.
 const char kDataReductionProxyExperiment[] = "data-reduction-proxy-experiment";
 
+// Gets the Chrome-Proxy "exp" directive value used by data reduction proxy to

[tbansal1, 2017/05/23 23:13:34]

rm "Gets" and update comment.

[RyanSturm, 2017/05/23 23:18:11]

Done.

+// receive an alternative back end implementation.
+const char kDataReductionProxyServerAlternative[] = "alternative";
+
 // The origin of the data reduction proxy fallback.
 const char kDataReductionProxyFallback[] = "spdy-proxy-auth-fallback";
 
 // The semicolon-separated list of proxy server URIs to override the list of
 // HTTP proxies returned by the Data Saver API. It is illegal to use
 // |kDataReductionProxy| or |kDataReductionProxyFallback| switch in conjunction
 // with |kDataReductionProxyHttpProxies|. If the URI omits a scheme, then the
 // proxy server scheme defaults to HTTP, and if the port is omitted then the
 // default port for that scheme is used. E.g. "http://foo.net:80",
 // "http://foo.net", "foo.net:80", and "foo.net" are all equivalent.
@@ skipping 48 matching lines @@
 // Enables sending a pageload metrics pingback after every page load.
 const char kEnableDataReductionProxyForcePingback[] =
     "enable-data-reduction-proxy-force-pingback";
 
 // Enables a 1 MB savings promo for the data reduction proxy.
 const char kEnableDataReductionProxySavingsPromo[] =
     "enable-data-reduction-proxy-savings-promo";
 
 }  // namespace switches
 }  // namespace data_reduction_proxy