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..dd842190ebd9d58acacc5c79ca430c3769011a15 100644
--- a/extensions/common/user_script.h
+++ b/extensions/common/user_script.h
@@ -317,9 +317,19 @@ class UserScript {
   bool incognito_enabled_;
 };
 
+// Information we need while removing scripts from a UserScriptLoader.
+struct UserScriptIDPair {

[Devlin, 2016/08/09 23:28:54]

Any reason to not use a std::pair?

[lazyboy, 2016/08/10 00:03:41]

I had that in the beginning, soon I had iter->second.second type stuff :) and
decided to use struct.

+  UserScriptIDPair(int id, const HostID& host_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