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

components/data_reduction_proxy/core/common/data_reduction_proxy_params.h

Index: components/data_reduction_proxy/core/common/data_reduction_proxy_params.h
diff --git a/components/data_reduction_proxy/core/common/data_reduction_proxy_params.h b/components/data_reduction_proxy/core/common/data_reduction_proxy_params.h
index d66f26130773da651bcb6db3bd8f6576c2037eb8..bd95d9213b6bcbffbdb0fcf814ffed6998af9b30 100644
--- a/components/data_reduction_proxy/core/common/data_reduction_proxy_params.h
+++ b/components/data_reduction_proxy/core/common/data_reduction_proxy_params.h
@@ -50,14 +50,18 @@ std::string GetLoFiFieldTrialName();
 // is force enabled through flags.
 std::string GetLoFiFlagFieldTrialName();
 
-// Returns true if this client is part of the "Enabled" group of the Lo-Fi field
-// trial.
+// Returns true if this client is part of the "Enabled" or "Enabled_Preview"
+// group of the Lo-Fi field trial, both of which mean Lo-Fi should be enabled.
 bool IsIncludedInLoFiEnabledFieldTrial();
 
 // Returns true if this client is part of the "Control" group of the Lo-Fi field
 // trial.
 bool IsIncludedInLoFiControlFieldTrial();
 
+// Returns true if this client is part of the "Preview" group of the Lo-Fi field
+// trial.
+bool IsIncludedInLoFiPreviewFieldTrial();
+
 // Returns true if this client has any of the values to enable Lo-Fi mode for
 // the "data-reduction-proxy-lo-fi" command line switch. This includes the
 // "always-on", "cellular-only", and "slow-connections-only" values.
@@ -79,6 +83,11 @@ bool IsLoFiSlowConnectionsOnlyViaFlags();
 // mode.
 bool IsLoFiDisabledViaFlags();
 
+// Returns true if this client has the command line switch to enable Lo-Fi
+// previews. This means a preview should be requested instead of placeholders
+// whenever Lo-Fi mode is on.
+bool AreLoFiPreviewsEnabledViaFlags();
+
 // Returns true if this client has the command line switch to show
 // interstitials for data reduction proxy bypasses.
 bool WarnIfNoDataReductionProxy();