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

For details see the STL Breaking Changes (VS14 CTP1) section of:
http://blogs.msdn.com/b/vcblog/archive/2014/06/06/
c-14-stl-features-fixes-and-breaking-changes-in-
visual-studio-14-ctp1.aspx

In particular:
"The Standard has always forbidden containers of const elements...
...elements must be Assignable, which const T isn't."

Error message is:
error C2338: The C++ Standard forbids containers of const
elements because allocator<const T> is ill-formed.

R=cpu@chromium.org
BUG=440500

Review URL: https://codereview.chromium.org/1066213002

Cr-Commit-Position: refs/heads/master@{#324339}

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 {