Making offline previews freshness configurable

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 5930a39df6823eb6a68afe8c4715eec797a16fd5..021db0a8de82139b99aa0c1b4b0c9159d582dde5 100644
--- a/components/previews/core/previews_experiments.h
+++ b/components/previews/core/previews_experiments.h
@@ -39,6 +39,10 @@ base::TimeDelta HostIndifferentBlackListPerHostDuration();
 // The amount of time after any opt out that no previews should be shown.
 base::TimeDelta SingleOptOutDuration();
 
+// The amount of time that an offline page is considered fresh enough to be
+// shown as a preview.
+base::TimeDelta OfflinePreviewFreshnessDuration();
+
 }  // namespace params
 
 enum class PreviewsType {