Add "q=preview" directive on main frame requests on poor networks

components/data_reduction_proxy/core/common/data_reduction_proxy_params.cc

Index: components/data_reduction_proxy/core/common/data_reduction_proxy_params.cc
diff --git a/components/data_reduction_proxy/core/common/data_reduction_proxy_params.cc b/components/data_reduction_proxy/core/common/data_reduction_proxy_params.cc
index 9530b2966159b39d01b15b422425acb3dd7192c0..ddfa34cc0cb94adaa7ae224cfc744e5b05418dc0 100644
--- a/components/data_reduction_proxy/core/common/data_reduction_proxy_params.cc
+++ b/components/data_reduction_proxy/core/common/data_reduction_proxy_params.cc
@@ -13,6 +13,7 @@
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_piece.h"
 #include "base/strings/string_split.h"
+#include "base/strings/string_util.h"
 #include "components/data_reduction_proxy/core/common/data_reduction_proxy_client_config_parser.h"
 #include "components/data_reduction_proxy/core/common/data_reduction_proxy_switches.h"
 #include "components/data_reduction_proxy/proto/client_config.pb.h"
@@ -27,6 +28,7 @@ namespace {
 
 const char kEnabled[] = "Enabled";
 const char kControl[] = "Control";
+const char kPreview[] = "Enabled_Preview";
 const char kDefaultSpdyOrigin[] = "https://proxy.googlezip.net:443";
 const char kDefaultQuicOrigin[] = "quic://proxy.googlezip.net:443";
 // A one-off change, until the Data Reduction Proxy configuration service is
@@ -83,13 +85,18 @@ std::string GetLoFiFlagFieldTrialName() {
 }
 
 bool IsIncludedInLoFiEnabledFieldTrial() {
-  return FieldTrialList::FindFullName(GetLoFiFieldTrialName()) == kEnabled;
+  std::string name = FieldTrialList::FindFullName(GetLoFiFieldTrialName());
+  return base::StartsWith(name, kEnabled, base::CompareCase::SENSITIVE);
 }
 
 bool IsIncludedInLoFiControlFieldTrial() {
   return FieldTrialList::FindFullName(GetLoFiFieldTrialName()) == kControl;
 }
 
+bool IsIncludedInLoFiPreviewFieldTrial() {
+  return FieldTrialList::FindFullName(GetLoFiFieldTrialName()) == kPreview;
+}
+
 bool IsLoFiOnViaFlags() {
   return IsLoFiAlwaysOnViaFlags() || IsLoFiCellularOnlyViaFlags() ||
          IsLoFiSlowConnectionsOnlyViaFlags();
@@ -127,6 +134,11 @@ bool IsLoFiDisabledViaFlags() {
          data_reduction_proxy::switches::kDataReductionProxyLoFiValueDisabled;
 }
 
+bool AreLoFiPreviewsEnabledViaFlags() {
+  return base::CommandLine::ForCurrentProcess()->HasSwitch(
+      data_reduction_proxy::switches::kEnableDataReductionProxyLoFiPreview);
+}
+
 bool WarnIfNoDataReductionProxy() {
   if (base::CommandLine::ForCurrentProcess()->HasSwitch(
           data_reduction_proxy::switches::