Select precache resources based on field trial experiment group

components/precache/core/precache_fetcher.cc

Index: components/precache/core/precache_fetcher.cc
diff --git a/components/precache/core/precache_fetcher.cc b/components/precache/core/precache_fetcher.cc
index eb9aeba0a15504b0b22cd635a144f74766f478f2..7db10e6a8e10278cd046d8b1d56819a166654aef 100644
--- a/components/precache/core/precache_fetcher.cc
+++ b/components/precache/core/precache_fetcher.cc
@@ -306,12 +306,14 @@ PrecacheFetcher::PrecacheFetcher(
     const GURL& config_url,
     const std::string& manifest_url_prefix,
     std::unique_ptr<PrecacheUnfinishedWork> unfinished_work,
+    uint32_t experiment_id,
     PrecacheFetcher::PrecacheDelegate* precache_delegate)
     : request_context_(request_context),
       config_url_(config_url),
       manifest_url_prefix_(manifest_url_prefix),
       precache_delegate_(precache_delegate),
-      pool_(kMaxParallelFetches) {
+      pool_(kMaxParallelFetches),
+      experiment_id_(experiment_id) {
   DCHECK(request_context_.get());  // Request context must be non-NULL.
   DCHECK(precache_delegate_);  // Precache delegate must be non-NULL.
 
@@ -521,8 +523,12 @@ void PrecacheFetcher::OnManifestFetchComplete(const Fetcher& source) {
       const int32_t len =
           std::min(manifest.resource_size(),
                    unfinished_work_->config_settings().top_resources_count());
+      const uint64_t resource_bitset = manifest.experiments()
+                                           .resources_by_experiment_group()
+                                           .at(experiment_id_)
+                                           .bitset();
       for (int i = 0; i < len; ++i) {
-        if (manifest.resource(i).has_url())
+        if (((0x1 << i) & resource_bitset) && manifest.resource(i).has_url())
           resource_urls_to_fetch_.push_back(GURL(manifest.resource(i).url()));
       }
     }