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..573d0b65672e19770f45d94c6b16f0b93c2186d8 100644
--- a/components/previews/core/previews_experiments.cc
+++ b/components/previews/core/previews_experiments.cc
@@ -179,6 +179,19 @@ bool IsPreviewsTypeEnabled(PreviewsType type) {
   }
 }
 
+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;