predictors: Put manifests for ResourcePrefetchPredictor behind a flag.

chrome/browser/predictors/resource_prefetch_predictor_unittest.cc

Index: chrome/browser/predictors/resource_prefetch_predictor_unittest.cc
diff --git a/chrome/browser/predictors/resource_prefetch_predictor_unittest.cc b/chrome/browser/predictors/resource_prefetch_predictor_unittest.cc
index 6e2e8cefc0b9c12ed866b32b18f652cb353f3e00..9f2a98bad54aee1e732bd8af694711539196b455 100644
--- a/chrome/browser/predictors/resource_prefetch_predictor_unittest.cc
+++ b/chrome/browser/predictors/resource_prefetch_predictor_unittest.cc
@@ -236,6 +236,7 @@ class ResourcePrefetchPredictorTest : public testing::Test {
     config.max_consecutive_misses = 2;
     config.min_resource_confidence_to_trigger_prefetch = 0.5;
     config.is_url_learning_enabled = true;
+    config.is_manifests_enabled = true;
 
     config.mode |= ResourcePrefetchPredictorConfig::LEARNING;
     predictor_.reset(new ResourcePrefetchPredictor(config, profile_.get()));