predictors: Put manifests for ResourcePrefetchPredictor behind a flag.

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 af03bee85c32e047ff876ead007764b31a369ae9..bee3ae2b145a2318709908c59bb8e7bc11237ce1 100644
--- a/chrome/browser/predictors/resource_prefetch_common.h
+++ b/chrome/browser/predictors/resource_prefetch_common.h
@@ -26,6 +26,7 @@ extern const char kLearningMode[];
 extern const char kExternalPrefetchingMode[];
 extern const char kPrefetchingMode[];
 extern const char kEnableUrlLearningParamName[];
+extern const char kEnableManifestsParamName[];
 extern const base::Feature kSpeculativeResourcePrefetchingFeature;
 
 struct ResourcePrefetchPredictorConfig;
@@ -126,6 +127,8 @@ struct ResourcePrefetchPredictorConfig {
   size_t max_prefetches_inflight_per_host_per_navigation;
   // True iff the predictor could use a url-based database.
   bool is_url_learning_enabled;
+  // True iff the predictor could use manifests.
+  bool is_manifests_enabled;
 };
 
 }  // namespace predictors