Adds PreviewsType version mechanism for clearing blacklist entries.

components/previews/core/previews_experiments.h

Index: components/previews/core/previews_experiments.h
diff --git a/components/previews/core/previews_experiments.h b/components/previews/core/previews_experiments.h
index ad0bded4a856ee7de386c5c329e28415dd0504a2..0a6f3a6f9a3864cbd5ea96a830600fd63d222f56 100644
--- a/components/previews/core/previews_experiments.h
+++ b/components/previews/core/previews_experiments.h
@@ -5,6 +5,10 @@
 #ifndef COMPONENTS_PREVIEWS_CORE_PREVIEWS_EXPERIMENTS_H_
 #define COMPONENTS_PREVIEWS_CORE_PREVIEWS_EXPERIMENTS_H_
 
+#include <memory>
+#include <utility>
+#include <vector>
+
 #include "base/time/time.h"
 #include "net/nqe/effective_connection_type.h"
 
@@ -53,9 +57,13 @@ net::EffectiveConnectionType EffectiveConnectionTypeThreshold();
 enum class PreviewsType {
   NONE = 0,
   OFFLINE = 1,
+  // Insert new enum values here. Keep values sequential to allow looping
+  // from NONE+1 to LAST-1.
   LAST = 2,
 };
 
+typedef std::vector<std::pair<PreviewsType, int>> PreviewsTypeList;
+
 // Returns true if any client-side previews experiment is active.
 bool IsIncludedInClientSidePreviewsExperimentsFieldTrial();
 
@@ -63,6 +71,13 @@ bool IsIncludedInClientSidePreviewsExperimentsFieldTrial();
 // prohibitvely slow networks is active.
 bool IsPreviewsTypeEnabled(PreviewsType type);
 
+// Returns the version of preview treatment |type|. Defaults to 0 if not
+// specified in field trial config.
+int GetPreviewsTypeVersion(PreviewsType type);
+
+// Returns the enabled PreviewsTypes with their version.
+std::unique_ptr<PreviewsTypeList> GetEnabledPreviews();
+
 // Sets the appropriate state for field trial and variations to imitate the
 // offline pages field trial.
 bool EnableOfflinePreviewsForTesting();