There's no need to export the key_iterator class, and exporting it

base/values.h

Index: base/values.h
===================================================================
--- base/values.h	(revision 83402)
+++ base/values.h	(working copy)
@@ -314,7 +314,7 @@
   // YOU SHOULD ALWAYS USE THE XXXWithoutPathExpansion() APIs WITH THESE, NOT
   // THE NORMAL XXX() APIs.  This makes sure things will work correctly if any
   // keys have '.'s in them.
-  class BASE_API key_iterator
+  class key_iterator
       : private std::iterator<std::input_iterator_tag, const std::string> {
    public:
     explicit key_iterator(ValueMap::const_iterator itr) { itr_ = itr; }