Reduce field type tracking to a single stable map per field.

test/cctest/test-migrations.cc

Index: test/cctest/test-migrations.cc
diff --git a/test/cctest/test-migrations.cc b/test/cctest/test-migrations.cc
index db03901c3dc0ca659b5990ee19158583fad54eeb..2f7ff8703c7f3187d6640732de6577fa167e18db 100644
--- a/test/cctest/test-migrations.cc
+++ b/test/cctest/test-migrations.cc
@@ -694,7 +694,7 @@ TEST(GeneralizeRepresentationHeapObjectToHeapObject) {
   Isolate* isolate = CcTest::i_isolate();
   Handle<HeapType> any_type = HeapType::Any(isolate);
 
-  const int kMaxClassesPerFieldType = 5;
+  const int kMaxClassesPerFieldType = 1;
   Handle<HeapType> current_type =
       HeapType::Class(Map::Create(isolate, 0), isolate);
 
@@ -1070,7 +1070,7 @@ TEST(ReconfigureDataFieldAttribute_GeneralizeRepresentationHeapObjToHeapObj) {
   Isolate* isolate = CcTest::i_isolate();
   Handle<HeapType> any_type = HeapType::Any(isolate);
 
-  const int kMaxClassesPerFieldType = 5;
+  const int kMaxClassesPerFieldType = 1;
   Handle<HeapType> current_type =
       HeapType::Class(Map::Create(isolate, 0), isolate);