Do fewer encoding checks in FlatStringReader used in the JSON stringifier.

src/json-stringifier.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_JSON_STRINGIFIER_H_
 #define V8_JSON_STRINGIFIER_H_
 
 #include "src/v8.h"
 
 #include "src/conversions.h"
@@ skipping 623 matching lines @@
   int worst_case_length = length << 3;
   if (builder_.CurrentPartCanFit(worst_case_length)) {
     DisallowHeapAllocation no_gc;
     Vector<const SrcChar> vector = GetCharVector<SrcChar>(string);
     IncrementalStringBuilder::NoExtendBuilder<DestChar> no_extend(
         &builder_, worst_case_length);
     SerializeStringUnchecked_(vector, &no_extend);
   } else {
     FlatStringReader reader(isolate_, string);
     for (int i = 0; i < reader.length(); i++) {
-      SrcChar c = static_cast<SrcChar>(reader.Get(i));
+      SrcChar c = reader.Get<SrcChar>(i);
       if (DoNotEscape(c)) {
         builder_.Append<SrcChar, DestChar>(c);
       } else {
         builder_.AppendCString(&JsonEscapeTable[c * kJsonEscapeTableEntrySize]);
       }
     }
   }
 
   builder_.Append<uint8_t, DestChar>('"');
 }
@@ skipping 42 matching lines @@
       SerializeString_<uint8_t, uc16>(object);
     } else {
       SerializeString_<uc16, uc16>(object);
     }
   }
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_STRINGIFIER_H_