Remove some non-essential StringPiece->std::string conversion in base/

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 b1f28b46214eff0d181b1134fb04098d0e025a03..9ce6a2c1e73fa7105402ee84580eb564bcddd700 100644
--- a/base/json/json_parser.cc
+++ b/base/json/json_parser.cc
@@ -151,9 +151,7 @@ class JSONStringValue : public Value {
     *out_value = UTF8ToUTF16(string_piece_);
     return true;
   }
-  Value* DeepCopy() const override {
-    return new StringValue(string_piece_.as_string());
-  }
+  Value* DeepCopy() const override { return new StringValue(string_piece_); }
   bool Equals(const Value* other) const override {
     std::string other_string;
     return other->IsType(TYPE_STRING) && other->GetAsString(&other_string) &&