predictors: Put manifests for ResourcePrefetchPredictor behind a flag.

chrome/browser/predictors/resource_prefetch_common.cc

Index: chrome/browser/predictors/resource_prefetch_common.cc
diff --git a/chrome/browser/predictors/resource_prefetch_common.cc b/chrome/browser/predictors/resource_prefetch_common.cc
index b48bf805120a905a1cc2631773db810fb8f9351f..ab9ec5da9d49cb0cf3ba0f0c19978d85c9e6fc4f 100644
--- a/chrome/browser/predictors/resource_prefetch_common.cc
+++ b/chrome/browser/predictors/resource_prefetch_common.cc
@@ -24,6 +24,7 @@ const char kLearningMode[] = "learning";
 const char kExternalPrefetchingMode[] = "external-prefetching";
 const char kPrefetchingMode[] = "prefetching";
 const char kEnableUrlLearningParamName[] = "enable-url-learning";
+const char kEnableManifestsParamName[] = "enable-manifests";
 
 const base::Feature kSpeculativeResourcePrefetchingFeature =
     base::Feature(kSpeculativeResourcePrefetchingFeatureName,
@@ -62,9 +63,13 @@ bool IsSpeculativeResourcePrefetchingEnabled(
   std::string enable_url_learning_value =
       base::GetFieldTrialParamValueByFeature(
           kSpeculativeResourcePrefetchingFeature, kEnableUrlLearningParamName);
-  if (enable_url_learning_value == "true") {
+  if (enable_url_learning_value == "true")
     config->is_url_learning_enabled = true;
-  }
+
+  std::string enable_manifests_value = base::GetFieldTrialParamValueByFeature(
+      kSpeculativeResourcePrefetchingFeature, kEnableManifestsParamName);
+  if (enable_manifests_value == "true")
+    config->is_manifests_enabled = true;
 
   // Ensure that a resource that was only seen once is never prefetched. This
   // prevents the easy mistake of trying to prefetch an ephemeral url.
@@ -137,8 +142,8 @@ ResourcePrefetchPredictorConfig::ResourcePrefetchPredictorConfig()
       min_resource_hits_to_trigger_prefetch(2),
       max_prefetches_inflight_per_navigation(5),
       max_prefetches_inflight_per_host_per_navigation(3),
-      is_url_learning_enabled(false) {
-}
+      is_url_learning_enabled(false),
+      is_manifests_enabled(false) {}
 
 ResourcePrefetchPredictorConfig::ResourcePrefetchPredictorConfig(
     const ResourcePrefetchPredictorConfig& other) = default;