Treat weak references in context weakly in write barrier.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index a973f12663683f16b2befb784f89fef84fc6facc..c7a9f7816027fd23f4fd62ba8503ace85584c285 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -251,8 +251,15 @@ static inline bool IsGrowStoreMode(KeyedAccessStoreMode store_mode) {
 enum IcCheckType { ELEMENT, PROPERTY };
 
 
-// Setter that skips the write barrier if mode is SKIP_WRITE_BARRIER.
-enum WriteBarrierMode { SKIP_WRITE_BARRIER, UPDATE_WRITE_BARRIER };
+// SKIP_WRITE_BARRIER skips the write barrier.
+// UPDATE_WEAK_WRITE_BARRIER skips the marking part of the write barrier and
+// only performs the generational part.
+// UPDATE_WRITE_BARRIER is doing the full barrier, marking and generational.
+enum WriteBarrierMode {
+  SKIP_WRITE_BARRIER,
+  UPDATE_WEAK_WRITE_BARRIER,
+  UPDATE_WRITE_BARRIER
+};
 
 
 // Indicates whether a value can be loaded as a constant.