Remove JsonPrefStore pruning of empty values on write.

base/values.h

Index: base/values.h
diff --git a/base/values.h b/base/values.h
index 3bc1f8bbafa2140f58d1557d0eb14c986e0e2faa..bffdbc7d37eb2e38a87608518bda37d2b4726dbe 100644
--- a/base/values.h
+++ b/base/values.h
@@ -324,6 +324,11 @@ class BASE_EXPORT DictionaryValue : public Value {
   virtual bool RemoveWithoutPathExpansion(const std::string& key,
                                           scoped_ptr<Value>* out_value);
 
+  // Removes a path, clearing out all dictionaries on |path| that remain empty
+  // after removing the value at |path|.
+  virtual bool RemovePath(const std::string& path,
+                          scoped_ptr<Value>* out_value);
+
   // Makes a copy of |this| but doesn't include empty dictionaries and lists in
   // the copy.  This never returns NULL, even if |this| itself is empty.
   DictionaryValue* DeepCopyWithoutEmptyChildren() const;