Use StringPiece more in base::Value interfaces.

base/json/json_parser.cc

Index: base/json/json_parser.cc
diff --git a/base/json/json_parser.cc b/base/json/json_parser.cc
index d97eccc96c999ded03c1760bb1a7b256434dfe36..36e9dcac736938c2e568fbe00c66dea8803f80b0 100644
--- a/base/json/json_parser.cc
+++ b/base/json/json_parser.cc
@@ -61,7 +61,7 @@ class DictionaryHiddenRootValue : public DictionaryValue {
   // Not overriding DictionaryValue::Remove because it just calls through to
   // the method below.
 
-  bool RemoveWithoutPathExpansion(const std::string& key,
+  bool RemoveWithoutPathExpansion(StringPiece key,
                                   std::unique_ptr<Value>* out) override {
     // If the caller won't take ownership of the removed value, just call up.
     if (!out)