Truncate strings synchronized.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 6d0f8d4e61110b55e641ad0b02dcf5dab36e2853..b7cb68d11f77ed17aa8807d8094628740c5aa52f 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -4331,8 +4331,10 @@ int HeapObject::SizeFromMap(Map* map) {
   }
   if (instance_type == ONE_BYTE_STRING_TYPE ||
       instance_type == ONE_BYTE_INTERNALIZED_STRING_TYPE) {
+    // Strings may get concurrently truncated, hence we have to access its
+    // length synchronized.
     return SeqOneByteString::SizeFor(
-        reinterpret_cast<SeqOneByteString*>(this)->length());
+        reinterpret_cast<SeqOneByteString*>(this)->synchronized_length());
   }
   if (instance_type == BYTE_ARRAY_TYPE) {
     return reinterpret_cast<ByteArray*>(this)->ByteArraySize();
@@ -4342,8 +4344,10 @@ int HeapObject::SizeFromMap(Map* map) {
   }
   if (instance_type == STRING_TYPE ||
       instance_type == INTERNALIZED_STRING_TYPE) {
+    // Strings may get concurrently truncated, hence we have to access its
+    // length synchronized.
     return SeqTwoByteString::SizeFor(
-        reinterpret_cast<SeqTwoByteString*>(this)->length());
+        reinterpret_cast<SeqTwoByteString*>(this)->synchronized_length());
   }
   if (instance_type == FIXED_DOUBLE_ARRAY_TYPE) {
     return FixedDoubleArray::SizeFor(