Adding UMA to track previews opt outs and blacklist eligibility

components/previews/core/previews_black_list.h

Index: components/previews/core/previews_black_list.h
diff --git a/components/previews/core/previews_black_list.h b/components/previews/core/previews_black_list.h
index ad192463f17372ea9b166b89fa643976b99d4584..e9151da625cf736a358f815ace26af04524eb465 100644
--- a/components/previews/core/previews_black_list.h
+++ b/components/previews/core/previews_black_list.h
@@ -30,6 +30,28 @@ class Clock;
 namespace previews {
 class PreviewsBlackListItem;
 
+enum class PreviewsEligibilityReason {
+  // The preview navigation was allowed.
+  ALLOWED = 0,
+  // The black list was not initialized.
+  BLACKLIST_UNAVAILABLE = 1,
+  // The black list has not loaded from disk yet.
+  BLACKLIST_DATA_NOT_LOADED = 2,
+  // The user has opted out of a preview recently.
+  USER_RECENTLY_OPTED_OUT = 3,
+  // The user has opted out of previews often, and is no longer shown previews
+  // on any host.
+  USER_BLACKLISTED = 4,
+  // The user has opted out of previews on a specific host often, and was not
+  // not shown a previews on that host.
+  HOST_BLACKLISTED = 5,
+  // The network quality estimate is not available.
+  NETWORK_QUALITY_UNAVAILABLE = 6,
+  // The network was fast enough to not warrant previews.
+  NETWORK_NOT_SLOW = 7,
+  LAST = 8,
+};
+
 // Manages the state of black listed domains for the previews experiment. Loads
 // the stored black list from |opt_out_store| and manages an in memory black
 // list on the IO thread. Updates to the black list are stored in memory and
@@ -58,9 +80,10 @@ class PreviewsBlackList {
   void AddPreviewNavigation(const GURL& url, bool opt_out, PreviewsType type);
 
   // Synchronously determines if |host_name| should be allowed to show previews.
-  // If the black list has loaded yet, this will always return false. |type| is
-  // not used to make this decision.
-  bool IsLoadedAndAllowed(const GURL& url, PreviewsType type) const;
+  // Returns the reason the blacklist disallowed the preview, or
+  // PreviewsEligibilityReason::ALLOWED if the preview is allowed.
+  PreviewsEligibilityReason IsLoadedAndAllowed(const GURL& url,
+                                               PreviewsType type) const;
 
   // Asynchronously deletes all entries in the in-memory black list. Informs
   // the backing store to delete entries between |begin_time| and |end_time|,