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

components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc

Index: components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc
diff --git a/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc b/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc
index 71879e4ece4eb4aebe9630091fde5df144977229..90356a060b7b048f2294ec9ad736f7c7bccc5cc9 100644
--- a/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc
+++ b/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc
@@ -26,6 +26,7 @@ const char kChromeProxyHeader[] = "chrome-proxy";
 const char kActionValueDelimiter = '=';
 
 const char kChromeProxyLoFiDirective[] = "q=low";
+const char kChromeProxyLoFiPreviewDirective[] = "q=preview";
 const char kChromeProxyLoFiExperimentDirective[] = "exp=lofi_active_control";
 
 const char kChromeProxyActionBlockOnce[] = "block-once";
@@ -61,6 +62,10 @@ const char* chrome_proxy_lo_fi_directive() {
   return kChromeProxyLoFiDirective;
 }
 
+const char* chrome_proxy_lo_fi_preview_directive() {
+  return kChromeProxyLoFiPreviewDirective;
+}
+
 const char* chrome_proxy_lo_fi_experiment_directive() {
   return kChromeProxyLoFiExperimentDirective;
 }