Remove unmarked entries from per context map caches.

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index fcf07092b77f2964c058bb39c3965fb43916c681..dbc13af01d4f9296a287c7d48e4d78cec3e7f3db 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -1168,6 +1168,12 @@ MUST_USE_RESULT static MaybeObject* UpdateMapCacheWith(Context* context,
     if (!maybe_result->ToObject(&result)) return maybe_result;
   }
   context->set_map_cache(MapCache::cast(result));
+  StatsCounter* counter =
+      Isolate::Current()->counters()->max_map_cache_length();
+  int length = FixedArray::cast(result)->length();
+  if (counter->Enabled() && (*counter->GetInternalPointer()) < length) {
+    counter->Set(length);
+  }
   return result;
 }
 
@@ -1191,7 +1197,11 @@ Handle<Map> Factory::ObjectLiteralMapFromCache(Handle<Context> context,
   Handle<MapCache> cache =
       Handle<MapCache>(MapCache::cast(context->map_cache()));
   Handle<Object> result = Handle<Object>(cache->Lookup(*keys));
-  if (result->IsMap()) return Handle<Map>::cast(result);
+  if (result->IsMap()) {
+    Isolate::Current()->counters()->map_cache_hits()->Increment();
+    return Handle<Map>::cast(result);
+  }
+  Isolate::Current()->counters()->map_cache_misses()->Increment();
   // Create a new map and add it to the cache.
   Handle<Map> map =
       CopyMap(Handle<Map>(context->object_function()->initial_map()),