Adds PreviewsType version mechanism for clearing blacklist entries.

components/previews/core/previews_experiments.cc

Index: components/previews/core/previews_experiments.cc
diff --git a/components/previews/core/previews_experiments.cc b/components/previews/core/previews_experiments.cc
index 3806bc9c8e8f6084023175e7ff2c222bed53c99f..8a425c7ad58309d2b911fcd4fe350f53a68f578d 100644
--- a/components/previews/core/previews_experiments.cc
+++ b/components/previews/core/previews_experiments.cc
@@ -179,6 +179,18 @@ bool IsPreviewsTypeEnabled(PreviewsType type) {
   }
 }
 
+std::vector<std::pair<PreviewsType, int>>* GetEnabledPreviews() {
+  std::vector<std::pair<PreviewsType, int>>* enabled_previews =
+      new std::vector<std::pair<PreviewsType, int>>();
+
+  // For now, all previews types have version 0.
+  // TODO(dougarnett): Add finch control of version for previews types.
+  if (IsPreviewsTypeEnabled(PreviewsType::OFFLINE)) {
+    enabled_previews->push_back({PreviewsType::OFFLINE, 0});
+  }
+  return enabled_previews;
+}
+
 bool EnableOfflinePreviewsForTesting() {
   std::map<std::string, std::string> params;
   params[kOfflinePagesSlowNetwork] = kExperimentEnabled;