Revert of Introduce HostID and de-couple Extensions from "script injection System" [browser side]

extensions/common/host_id.h

-// Copyright 2015 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 EXTENSIONS_COMMON_HOST_ID_H_
-#define EXTENSIONS_COMMON_HOST_ID_H_
-
-#include <string>
-
-// IDs of hosts who own user scripts.
-// A HostID is immutable after creation.
-struct HostID {
-  enum HostType { EXTENSIONS, WEBUI };
-
-  HostID() {}
-  HostID(HostType type, const std::string& id)
-      : type_(type), id_(id) {}
-
-  bool operator<(const HostID& host_id) const {
-    if (type_ != host_id.type())
-      return type_ < host_id.type();
-    else if (id_ != host_id.id())
-      return id_ < host_id.id();
-    return false;
-  }
-
-  HostType type() const { return type_; }
-  const std::string& id() const { return id_; }
-
- private:
-  // The type of the host.
-  HostType type_;
-
-  // Similar to extension_id, host_id is a unique indentifier for a host,
-  // e.g., an Extension or WebUI.
-  std::string id_;
-};
-
-#endif  // EXTENSIONS_COMMON_HOST_ID_H_