PrototypeInfo.validity_cell: adding comment

BUG=

Review URL: https://codereview.chromium.org/1370443002

Cr-Commit-Position: refs/heads/master@{#30915}

src/objects.h

@@ -5990,6 +5990,10 @@ class PrototypeInfo : public Struct {
   inline void set_registry_slot(int slot);
   // [validity_cell]: Cell containing the validity bit for prototype chains
   // going through this object, or Smi(0) if uninitialized.
+  // When a prototype object changes its map, then both its own validity cell
+  // and those of all "downstream" prototypes are invalidated; handlers for a
+  // given receiver embed the currently valid cell for that receiver's prototype
+  // during their compilation and check it on execution.
   DECL_ACCESSORS(validity_cell, Object)
   // [constructor_name]: User-friendly name of the original constructor.
   DECL_ACCESSORS(constructor_name, Object)