predictors: Put manifests for ResourcePrefetchPredictor behind a flag.

chrome/browser/predictors/resource_prefetch_predictor.cc

Index: chrome/browser/predictors/resource_prefetch_predictor.cc
diff --git a/chrome/browser/predictors/resource_prefetch_predictor.cc b/chrome/browser/predictors/resource_prefetch_predictor.cc
index 7b9979906fb4a7a074304f966c5304088ed5fcee..06b17c95fcc59053f05e9d8ce4411e0d6632206f 100644
--- a/chrome/browser/predictors/resource_prefetch_predictor.cc
+++ b/chrome/browser/predictors/resource_prefetch_predictor.cc
@@ -1467,7 +1467,8 @@ void ResourcePrefetchPredictor::OnManifestFetched(
   if (initialization_state_ != INITIALIZED)
     return;
 
-  if (host.length() > ResourcePrefetchPredictorTables::kMaxStringLength ||
+  if (!config_.is_manifests_enabled ||
+      host.length() > ResourcePrefetchPredictorTables::kMaxStringLength ||
       static_cast<uint32_t>(manifest.ByteSize()) > kMaxManifestByteSize) {
     return;
   }