predictors: Move the configuration of speculative prefetch to field trials.

chrome/browser/predictors/resource_prefetch_common.h

Index: chrome/browser/predictors/resource_prefetch_common.h
diff --git a/chrome/browser/predictors/resource_prefetch_common.h b/chrome/browser/predictors/resource_prefetch_common.h
index 46e9707d488ea3f57c03100fec0df0f341dbbde8..3ec303d25d186fc7eced89b76116017943ed572b 100644
--- a/chrome/browser/predictors/resource_prefetch_common.h
+++ b/chrome/browser/predictors/resource_prefetch_common.h
@@ -7,6 +7,7 @@
 
 #include <stddef.h>
 
+#include "base/feature_list.h"
 #include "base/time/time.h"
 #include "components/sessions/core/session_id.h"
 #include "url/gurl.h"
@@ -19,6 +20,12 @@ class WebContents;
 
 namespace predictors {
 
+extern const char kSpeculativeResourcePrefetchingFeatureName[];
+extern const char kModeParamName[];
+extern const char kLearningMode[];
+extern const char kExternalPrefetchingMode[];
+extern const char kPrefetchingMode[];
+
 struct ResourcePrefetchPredictorConfig;
 
 // Returns true if prefetching is enabled. And will initilize the |config|