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

extensions/common/host_id.h

Index: extensions/common/host_id.h
diff --git a/extensions/common/host_id.h b/extensions/common/host_id.h
new file mode 100644
index 0000000000000000000000000000000000000000..004480704f07766662006c7c3c570883c7e9a296
--- /dev/null
+++ b/extensions/common/host_id.h
@@ -0,0 +1,39 @@
+// 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_