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 021db0a8de82139b99aa0c1b4b0c9159d582dde5..5ba776f68f30754ebc2f7f176b32b9511af94cf4 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"
 
 namespace previews {
@@ -58,6 +62,13 @@ bool IsIncludedInClientSidePreviewsExperimentsFieldTrial();
 // prohibitvely slow networks is active.
 bool IsPreviewsTypeEnabled(PreviewsType type);
 
+// Returns the version of preview treatment |type| to use if it is enabled.
+// Should only be called if |IsPreviewsTypeEnabled(type)| returns true.
+bool IsPreviewsTypeEnabled(PreviewsType type);
+
+// Returns the enabled PreviewsTypes with their version.
+std::unique_ptr<std::vector<std::pair<PreviewsType, int>>> GetEnabledPreviews();
+
 // Sets the appropriate state for field trial and variations to imitate the
 // offline pages field trial.
 bool EnableOfflinePreviewsForTesting();