mojo: Switch the clipboard interface over to using map<>.

mojo/public/cpp/bindings/map.h

Index: mojo/public/cpp/bindings/map.h
diff --git a/mojo/public/cpp/bindings/map.h b/mojo/public/cpp/bindings/map.h
index f09937a40046c98e9b5f2f1cb5e684191957af6b..f68d074dea77f544e4cb3c2696da33bd940988c1 100644
--- a/mojo/public/cpp/bindings/map.h
+++ b/mojo/public/cpp/bindings/map.h
@@ -88,6 +88,10 @@ class Map {
     return Traits::at(&map_, key);
   }
 
+  ValueRefType operator[](KeyForwardType key) {
+    return Traits::GetOrInsert(&map_, key);
+  }
+
   void Swap(Map<Key, Value>* other) {
     std::swap(is_null_, other->is_null_);
     map_.swap(other->map_);