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..4db8fc3be8ec89e7a71394b989948b7749330db7 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"
 
@@ -63,6 +67,13 @@ 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.

[bengr, 2017/02/09 17:33:47]

Why? Isn't a version independent of whether it is enabled?

[dougarnett, 2017/02/13 17:56:17]

Done.

+int GetPreviewsTypeVersion(PreviewsType type);
+
+// Returns the enabled PreviewsTypes with their version.
+std::unique_ptr<std::vector<std::pair<PreviewsType, int>>> GetEnabledPreviews();

[bengr, 2017/02/09 17:33:47]

Might be more readable if you define a PreviewsTypeList

[dougarnett, 2017/02/13 17:56:17]

Done.

+
 // Sets the appropriate state for field trial and variations to imitate the
 // offline pages field trial.
 bool EnableOfflinePreviewsForTesting();