Add struct traits for base::Value.

base/values.cc

Index: base/values.cc
diff --git a/base/values.cc b/base/values.cc
index fd0bbe17a41468da40b3e55bc64c50ef3be5415d..ca3692deccae32ff401b99ac35487db78dc66199 100644
--- a/base/values.cc
+++ b/base/values.cc
@@ -118,6 +118,10 @@ bool Value::GetAsString(const StringValue** out_value) const {
   return false;
 }
 
+bool Value::GetAsString(StringPiece* out_value) const {
+  return false;
+}
+
 bool Value::GetAsList(ListValue** out_value) {
   return false;
 }
@@ -287,6 +291,12 @@ bool StringValue::GetAsString(const StringValue** out_value) const {
   return true;
 }
 
+bool StringValue::GetAsString(StringPiece* out_value) const {
+  if (out_value)
+    *out_value = value_;
+  return true;
+}
+
 StringValue* StringValue::DeepCopy() const {
   return new StringValue(value_);
 }