Adds PreviewsType version mechanism for clearing blacklist entries.

components/previews/core/previews_experiments.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/previews/core/previews_experiments.h"
 
 #include <map>
 #include <string>
 
 #include "base/logging.h"
@@ skipping 161 matching lines @@
 bool IsPreviewsTypeEnabled(PreviewsType type) {
   switch (type) {
     case PreviewsType::OFFLINE:
       return ParamValue(kOfflinePagesSlowNetwork) == kExperimentEnabled;
     default:
       NOTREACHED();
       return false;
   }
 }
 
+std::unique_ptr<std::vector<std::pair<PreviewsType, int>>>
+GetEnabledPreviews() {
+  std::unique_ptr<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.

[RyanSturm, 2017/01/25 23:17:53]

I'd add the offline previews field trial support in this CL with something like
"Offline.Version":"x" and default to 0 like you are doing.

I think generally, it could follow the pattern:

"<preview_name>.Version" as the parameter name. WDYT?

If you want to push this off to a later CL, add a bug to the TODO.

[dougarnett, 2017/01/26 00:48:26]

Add param but followed current naming style. Please check that style. 

Let men know if we can and you think we should reconcile the param name
"show_offline_pages" with the PreviewsType::OFFLINE name.

+  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;
   return variations::AssociateVariationParams(kClientSidePreviewsFieldTrial,
                                               kEnabled, params) &&
          base::FieldTrialList::CreateFieldTrial(kClientSidePreviewsFieldTrial,
                                                 kEnabled);
 }
 
 }  // namespace previews