Improve SeqString::Truncate for latest allocated strings.

src/json-parser.h

Index: src/json-parser.h
diff --git a/src/json-parser.h b/src/json-parser.h
index 28ef8b33c8be019fb465830650d48bf41c40b23b..be0c86a7b035fe9fd01db25dac656f754935863c 100644
--- a/src/json-parser.h
+++ b/src/json-parser.h
@@ -580,22 +580,16 @@ Handle<String> JsonParser<seq_ascii>::SlowScanJsonString(
       Advance();
     }
   }
-  // Shrink seq_string length to count.
-  if (isolate()->heap()->InNewSpace(*seq_str)) {
-    isolate()->heap()->new_space()->
-        template ShrinkStringAtAllocationBoundary<StringType>(
-            *seq_str, count);
-  } else {
-    int string_size = StringType::SizeFor(count);
-    int allocated_string_size = StringType::SizeFor(length);
-    int delta = allocated_string_size - string_size;
-    Address start_filler_object = seq_str->address() + string_size;
-    seq_str->set_length(count);
-    isolate()->heap()->CreateFillerObjectAt(start_filler_object, delta);
-  }
+
   ASSERT_EQ('"', c0_);
   // Advance past the last '"'.
   AdvanceSkipWhitespace();
+
+  // Shrink seq_string length to count and return.

[Hannes Payer (out of office), 2013/03/21 12:48:30]

See comment in src/objects.h. I would let Truncate return the right string
object, i.e. the empty string in the new_length==0 and the original string
object otherwise.

+  if (count == 0) {
+    return isolate_->factory()->empty_string();
+  }
+  SeqString::cast(*seq_str)->Truncate(count);
   return seq_str;
 }
 
@@ -623,8 +617,8 @@ Handle<String> JsonParser<seq_ascii>::ScanJsonString() {
         int beg_pos = position_;
         position_ = position;
         return SlowScanJsonString<SeqOneByteString, uint8_t>(source_,
-                                                            beg_pos,
-                                                            position_);
+                                                             beg_pos,
+                                                             position_);
       }
       if (c0 < 0x20) return Handle<String>::null();
       if (static_cast<uint32_t>(c0) >