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

chrome/browser/tab_contents/origins_seen_service.h

Index: chrome/browser/tab_contents/origins_seen_service.h
diff --git a/chrome/browser/tab_contents/origins_seen_service.h b/chrome/browser/tab_contents/origins_seen_service.h
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+++ b/chrome/browser/tab_contents/origins_seen_service.h
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+// 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);
+
+ private:
+  // Used by |HaveAlreadySeenOrigin|.
+  base::MRUCache<url::Origin, bool> origins_seen_;
+};
+
+#endif  // CHROME_BROWSER_TAB_CONTENTS_ORIGINS_SEEN_SERVICE_H_