Revert 23058 - "Allow externalizing cons and sliced string."

Reason for revert: broke inspector

| Fulfill overdue TODO.
|
| R=mstarzinger@chromium.org
|
| Review URL: https://codereview.chromium.org/460643002

TBR=yangguo@chromium.org,mstarzinger@chromium.org
BUG=403521
LOG=n

Review URL: https://codereview.chromium.org/470913002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23115 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/api.cc

@@ -5551,6 +5551,11 @@ bool v8::String::CanMakeExternal() {
   i::Handle<i::String> obj = Utils::OpenHandle(this);
   i::Isolate* isolate = obj->GetIsolate();
 
+  // TODO(yangguo): Externalizing sliced/cons strings allocates.
+  // This rule can be removed when all code that can
+  // trigger an access check is handlified and therefore GC safe.
+  if (isolate->heap()->old_pointer_space()->Contains(*obj)) return false;
+
   if (isolate->string_tracker()->IsFreshUnusedString(obj)) return false;
   int size = obj->Size();  // Byte size of the original string.
   if (size < i::ExternalString::kShortSize) return false;