Retain maps embedded in optimized code for several garbage collections.

src/lithium.cc

Index: src/lithium.cc
diff --git a/src/lithium.cc b/src/lithium.cc
index c15e9dbee2612f8e670a036a42357e76302c725e..e9991cb81e0c777002556b78af3e66c20dc63588 100644
--- a/src/lithium.cc
+++ b/src/lithium.cc
@@ -448,6 +448,10 @@ void LChunk::RegisterWeakObjectsInOptimizedCode(Handle<Code> code) const {
     }
   }
   for (int i = 0; i < maps.length(); i++) {
+    if (maps.at(i)->dependent_code()->number_of_entries(
+            DependentCode::kWeakCodeGroup) == 0) {
+      isolate()->heap()->AddRetainedMap(maps.at(i));
+    }
     Map::AddDependentCode(maps.at(i), DependentCode::kWeakCodeGroup, code);
   }
   for (int i = 0; i < objects.length(); i++) {