Rework some UserScriptLoader logic in preparation to removing UserScript copy.

extensions/common/user_script.h

Index: extensions/common/user_script.h
diff --git a/extensions/common/user_script.h b/extensions/common/user_script.h
index d6b33770f7c5833b9f30bf26246afa270b38d7e5..44a2d671f08e914bdecbae317562438fe37477e4 100644
--- a/extensions/common/user_script.h
+++ b/extensions/common/user_script.h
@@ -317,9 +317,20 @@ class UserScript {
   bool incognito_enabled_;
 };
 
+// Information we need while removing scripts from a UserScriptLoader.
+struct UserScriptIDPair {
+  UserScriptIDPair(int id, const HostID& host_id);
+  UserScriptIDPair(int id);
+
+  int id;
+  HostID host_id;
+};
+
 // For storing UserScripts with unique IDs in sets.
 bool operator<(const UserScript& script1, const UserScript& script2);
 
+bool operator<(const UserScriptIDPair& a, const UserScriptIDPair& b);
+
 typedef std::vector<UserScript> UserScriptList;
 
 }  // namespace extensions