Fix illegal use of const in container contents to unbreak VS 2015.

sandbox/win/src/handle_closer.h

Index: sandbox/win/src/handle_closer.h
diff --git a/sandbox/win/src/handle_closer.h b/sandbox/win/src/handle_closer.h
index 60473b3ea00f0e25a28c6ec7c7b54005638817bc..fa656953ccc256db7f88bd516e329c8442fbde8c 100644
--- a/sandbox/win/src/handle_closer.h
+++ b/sandbox/win/src/handle_closer.h
@@ -19,8 +19,7 @@ namespace sandbox {
 // This is a map of handle-types to names that we need to close in the
 // target process. A null set means we need to close all handles of the
 // given type.
-typedef std::map<const base::string16, std::set<const base::string16> >
-    HandleMap;
+typedef std::map<const base::string16, std::set<base::string16> > HandleMap;
 
 // Type and set of corresponding handle names to close.
 struct HandleListEntry {