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..880dd1e16e4680716c5d0e14072d5167f1937eaf 100644
--- a/components/previews/core/previews_experiments.h
+++ b/components/previews/core/previews_experiments.h
@@ -5,6 +5,8 @@
 #ifndef COMPONENTS_PREVIEWS_CORE_PREVIEWS_EXPERIMENTS_H_
 #define COMPONENTS_PREVIEWS_CORE_PREVIEWS_EXPERIMENTS_H_
 
+#include <vector>
+
 #include "base/time/time.h"
 
 namespace previews {
@@ -58,6 +60,10 @@ bool IsIncludedInClientSidePreviewsExperimentsFieldTrial();
 // prohibitvely slow networks is active.
 bool IsPreviewsTypeEnabled(PreviewsType type);
 
+// Returns the enabled PreviewsTypes with their version.
+// Caller assumes ownership of returned vector.

[tbansal1, 2017/01/23 20:06:11]

In Chromium, if a function returns raw ptr, the default is to assume that the
callee has the ownership. Two possible alternate solutions: (i) return
unique_ptr, (ii)  take the raw ptr as an argument to the method.

[dougarnett, 2017/01/24 00:52:38]

Done.

+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();