Layout descriptor must be trimmed when corresponding descriptors array is trimmed to stay in sync.

src/layout-descriptor.h

Index: src/layout-descriptor.h
diff --git a/src/layout-descriptor.h b/src/layout-descriptor.h
index 8f2942c0eb52319b2ca1b61fe1f6fad86a840912..0a14f53198f5c4fd48dd78b28d4b33d5d8792526 100644
--- a/src/layout-descriptor.h
+++ b/src/layout-descriptor.h
@@ -70,7 +70,14 @@ class LayoutDescriptor : public FixedTypedArray<Uint32ArrayTraits> {
   V8_INLINE static LayoutDescriptor* FastPointerLayout();
 
   // Check that this layout descriptor corresponds to given map.
-  bool IsConsistentWithMap(Map* map);
+  bool IsConsistentWithMap(Map* map, bool check_tail = false);
+
+  // Trims this layout descriptor to given number of descriptors. This happens
+  // only when corresponding descriptors array is trimmed.
+  // The layout descriptor could be trimmed if it was slow or it could
+  // become fast.
+  LayoutDescriptor* Trim(Heap* heap, Map* map, DescriptorArray* descriptors,
+                         int num_descriptors);
 
 #ifdef OBJECT_PRINT
   // For our gdb macros, we should perhaps change these in the future.
@@ -94,6 +101,21 @@ class LayoutDescriptor : public FixedTypedArray<Uint32ArrayTraits> {
   V8_INLINE static bool InobjectUnboxedField(int inobject_properties,
                                              PropertyDetails details);
 
+  // Calculates minimal layout descriptor capacity required for given
+  // |map|, |descriptors| and |num_descriptors|.
+  V8_INLINE static int CalculateCapacity(Map* map, DescriptorArray* descriptors,
+                                         int num_descriptors);
+
+  // Calculates the length of the slow-mode backing store array by given layout
+  // descriptor length.
+  V8_INLINE static int GetSlowModeBackingStoreLength(int length);
+
+  // Fills in clean |layout_descriptor| according to given |map|, |descriptors|
+  // and |num_descriptors|.
+  V8_INLINE static LayoutDescriptor* Initialize(
+      LayoutDescriptor* layout_descriptor, Map* map,
+      DescriptorArray* descriptors, int num_descriptors);
+
   static Handle<LayoutDescriptor> EnsureCapacity(
       Isolate* isolate, Handle<LayoutDescriptor> layout_descriptor,
       int new_capacity);