Adds PreviewsType version mechanism for clearing blacklist entries.

components/previews/core/previews_experiments.cc

Index: components/previews/core/previews_experiments.cc
diff --git a/components/previews/core/previews_experiments.cc b/components/previews/core/previews_experiments.cc
index 3806bc9c8e8f6084023175e7ff2c222bed53c99f..fdd1d4ebcd2167ad374b7d07f57fa4a67d55d9ad 100644
--- a/components/previews/core/previews_experiments.cc
+++ b/components/previews/core/previews_experiments.cc
@@ -25,6 +25,9 @@ const char kEnabled[] = "Enabled";
 // Allow offline pages to show for prohibitively slow networks.
 const char kOfflinePagesSlowNetwork[] = "show_offline_pages";
 
+// Version of treatment for showing offline pages, if enabled.
+const char kOfflinePagesSlowNetworkVersion[] = "show_offline_pages.version";
+
 // The maximum number of recent previews navigations the black list looks at to
 // determine if a host is blacklisted.
 const char kMaxStoredHistoryLengthPerHost[] =
@@ -179,6 +182,31 @@ bool IsPreviewsTypeEnabled(PreviewsType type) {
   }
 }
 
+bool GetPreviewsTypeVersion(PreviewsType type) {
+  DCHECK(IsPreviewsTypeEnabled(type));
+  int version = 0;  // default
+  switch (type) {
+    case PreviewsType::OFFLINE:
+      base::StringToInt(ParamValue(kOfflinePagesSlowNetworkVersion), &version);
+      return version;

[tbansal1, 2017/01/26 00:59:14]

I think the switch-case can be rewritten to get compile-time checks:

switch (type) {
 case PreviewsType::OFFLINE:
   return blah;
 case PreviewsType::NONE:
 case PreviewsType::OFFLINE:
        NOTREACHED();
       return 0;
}

See
https://groups.google.com/a/chromium.org/forum/#!searchin/chromium-dev/switch...

[dougarnett, 2017/02/07 21:15:02]

Done.

+    default:
+      NOTREACHED();
+  }
+  return version;
+}
+
+std::unique_ptr<std::vector<std::pair<PreviewsType, int>>>
+GetEnabledPreviews() {
+  std::unique_ptr<std::vector<std::pair<PreviewsType, int>>> enabled_previews(
+      new std::vector<std::pair<PreviewsType, int>>());
+
+  if (IsPreviewsTypeEnabled(PreviewsType::OFFLINE)) {
+    enabled_previews->push_back(
+        {PreviewsType::OFFLINE, GetPreviewsTypeVersion(PreviewsType::OFFLINE)});
+  }
+  return enabled_previews;
+}
+
 bool EnableOfflinePreviewsForTesting() {
   std::map<std::string, std::string> params;
   params[kOfflinePagesSlowNetwork] = kExperimentEnabled;