predictors: Add an about:flags flag for speculative prefetch.

chrome/browser/predictors/resource_prefetch_common.h

 // 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.
 
 #ifndef CHROME_BROWSER_PREDICTORS_RESOURCE_PREFETCH_COMMON_H_
 #define CHROME_BROWSER_PREDICTORS_RESOURCE_PREFETCH_COMMON_H_
 
 #include <stddef.h>
 
+#include "base/feature_list.h"
 #include "base/time/time.h"
 #include "components/sessions/core/session_id.h"
 #include "url/gurl.h"
 
 class Profile;
 
 namespace content {
 class WebContents;
 }
 
 namespace predictors {
 
 extern const char kSpeculativeResourcePrefetchingFeatureName[];
 extern const char kModeParamName[];
 extern const char kLearningMode[];
 extern const char kExternalPrefetchingMode[];
 extern const char kPrefetchingMode[];
 extern const char kEnableUrlLearningParamName[];
+extern const base::Feature kSpeculativeResourcePrefetchingFeature;
 
 struct ResourcePrefetchPredictorConfig;
 
 // Returns true if prefetching is enabled. And will initilize the |config|
 // fields to the appropritate values.
 bool IsSpeculativeResourcePrefetchingEnabled(
     Profile* profile,
     ResourcePrefetchPredictorConfig* config);
 
 // Represents the type of key based on which prefetch data is stored.
@@ skipping 84 matching lines @@
   // Maximum number of prefetches that can be inflight for a host for a single
   // navigation.
   size_t max_prefetches_inflight_per_host_per_navigation;
   // True iff the predictor could use a url-based database.
   bool is_url_learning_enabled;
 };
 
 }  // namespace predictors
 
 #endif  // CHROME_BROWSER_PREDICTORS_RESOURCE_PREFETCH_COMMON_H_