Revert "Added ability to lock strings to prevent their representation or encoding from changing."

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 0e1ae4484948d806817af5b66ba71ae65f0f5802..1f52a225de9ceee3a4cefc1d0e1d1e8b785da2e4 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -9139,22 +9139,13 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_ParseJson) {
   ASSERT_EQ(1, args.length());
   CONVERT_ARG_CHECKED(String, source, 0);
 
-  source = FlattenGetString(source);
-  ASSERT(!source->IsConsString());
-  ASSERT(!source->IsSlicedString());
+  source = Handle<String>(source->TryFlattenGetString());
+  // Optimized fast case where we only have ascii characters.
   Handle<Object> result;
-  {
-    StringLock lock_representation(source);
-    if (source->IsSeqAsciiString()) {
-      result = JsonParser<SeqAsciiString>::Parse(source);
-    } else if (source->IsExternalTwoByteString()) {
-      result = JsonParser<ExternalTwoByteString>::Parse(source);
-    } else if (source->IsSeqTwoByteString()) {
-      result = JsonParser<SeqTwoByteString>::Parse(source);
-    } else {
-      ASSERT(source->IsExternalAsciiString());
-      result = JsonParser<ExternalAsciiString>::Parse(source);
-    }
+  if (source->IsSeqAsciiString()) {
+    result = JsonParser<true>::Parse(source);
+  } else {
+    result = JsonParser<false>::Parse(source);
   }
   if (result.is_null()) {
     // Syntax error or stack overflow in scanner.