Remove unmarked entries from per context map caches.

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index da6e0887576b3a3cc3413423b798b29e26c0fbe7..0f3d44cc25f0d9bc10a2b137bf69f5d7a9022759 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -225,7 +225,6 @@ class Context: public FixedArray {
     OPAQUE_REFERENCE_FUNCTION_INDEX,
     CONTEXT_EXTENSION_FUNCTION_INDEX,
     OUT_OF_MEMORY_INDEX,
-    MAP_CACHE_INDEX,
     CONTEXT_DATA_INDEX,
     ALLOW_CODE_GEN_FROM_STRINGS_INDEX,
     DERIVED_GET_TRAP_INDEX,
@@ -234,6 +233,7 @@ class Context: public FixedArray {
     // Properties from here are treated as weak references by the full GC.
     // Scavenge treats them as strong references.
     OPTIMIZED_FUNCTIONS_LIST,  // Weak.
+    MAP_CACHE_INDEX,  // Weak.
     NEXT_CONTEXT_LINK,  // Weak.
 
     // Total number of slots.