Add struct traits for base::Value.

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 1850a1a3f66e2dc4814235c75d3b8190bef794f9..ab5c061f8ead844bbf7ecfc7fb2df637e33ce8c7 100644
--- a/base/json/json_parser.cc
+++ b/base/json/json_parser.cc
@@ -151,6 +151,10 @@ class JSONStringValue : public Value {
     *out_value = UTF8ToUTF16(string_piece_);
     return true;
   }
+  bool GetAsString(StringPiece* out_value) const override {
+    *out_value = string_piece_;
+    return true;
+  }
   Value* DeepCopy() const override { return new StringValue(string_piece_); }
   bool Equals(const Value* other) const override {
     std::string other_string;