Retire StringTracker.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 2b874ac4885e2c84c61e0d6cf88556c120435f0c..6bc7f5f15ab6e30e76cc6a2ddebe94a50456c348 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -5132,7 +5132,6 @@ static inline int WriteHelper(const String* string,
   ENTER_V8(isolate);
   DCHECK(start >= 0 && length >= -1);
   i::Handle<i::String> str = Utils::OpenHandle(string);
-  isolate->string_tracker()->RecordWrite(str);
   if (options & String::HINT_MANY_WRITES_EXPECTED) {
     // Flatten the string for efficiency.  This applies whether we are
     // using StringCharacterStream or Get(i) to access the characters.
@@ -5838,9 +5837,6 @@ bool v8::String::MakeExternal(v8::String::ExternalStringResource* resource) {
     return false;  // Already an external string.
   }
   ENTER_V8(isolate);
-  if (isolate->string_tracker()->IsFreshUnusedString(obj)) {
-    return false;
-  }
   if (isolate->heap()->IsInGCPostProcessing()) {
     return false;
   }
@@ -5865,9 +5861,6 @@ bool v8::String::MakeExternal(
     return false;  // Already an external string.
   }
   ENTER_V8(isolate);
-  if (isolate->string_tracker()->IsFreshUnusedString(obj)) {
-    return false;
-  }
   if (isolate->heap()->IsInGCPostProcessing()) {
     return false;
   }
@@ -5888,9 +5881,10 @@ bool v8::String::CanMakeExternal() {
   i::Handle<i::String> obj = Utils::OpenHandle(this);
   i::Isolate* isolate = obj->GetIsolate();
 
-  if (isolate->string_tracker()->IsFreshUnusedString(obj)) return false;
+  // Old space strings should be externalized.
+  if (!isolate->heap()->new_space()->Contains(*obj)) return true;
   int size = obj->Size();  // Byte size of the original string.
-  if (size < i::ExternalString::kShortSize) return false;
+  if (size <= i::ExternalString::kShortSize) return false;
   i::StringShape shape(*obj);
   return !shape.IsExternal();
 }