Histogram the scheme of an origin on the 1st navigation to it.

chrome/browser/tab_contents/origins_seen_service.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_TAB_CONTENTS_ORIGINS_SEEN_SERVICE_H_
+#define CHROME_BROWSER_TAB_CONTENTS_ORIGINS_SEEN_SERVICE_H_
+
+#include "base/containers/mru_cache.h"
+#include "components/keyed_service/core/keyed_service.h"
+#include "url/origin.h"
+
+class OriginsSeenService : public KeyedService {
+ public:
+  OriginsSeenService();
+  ~OriginsSeenService() override;
+
+  // Used when deciding whether or not to record
+  // Navigation.SchemePerUniqueOrigin[OTR]. Inserts a copy of |origin| into the
+  // set |origins_seen_|, and returns whether or not |origin| was already in the
+  // set.
+  bool HaveAlreadySeenOrigin(const url::Origin& origin);

[battre, 2016/04/18 15:20:51]

Nit (up to you): The function name suggests that this is a getter and the
comment explains the opposite. How about

bool InsertSeenOrigin(const url::Origin& origin);

or just

bool Insert(const url::Origin& origin);

The call could be still quite readable:

bool already_seen = origins_seen_service->Insert(original);

[palmer, 2016/04/18 20:12:44]

Done.

+
+ private:
+  // Used by |HaveAlreadySeenOrigin|.
+  base::MRUCache<url::Origin, bool> origins_seen_;

[battre, 2016/04/18 15:20:51]

Can you add a comment that this is intentionally in volatile storage because the
OriginsSeenService is also instantiated for OTR profiles.

[palmer, 2016/04/18 20:12:43]

Done.

+};
+
+#endif  // CHROME_BROWSER_TAB_CONTENTS_ORIGINS_SEEN_SERVICE_H_