Fix WeakMap processing for evacuation candidates.

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index b4f488bd9570e98191da12d61dc9ee25b3dfa084..fb5a7e7caef741e1c2fd1bd51c4a95662ba32417 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -337,6 +337,7 @@ void MarkCompactCollector::VerifyMarkbitsAreClean() {
   for (HeapObject* obj = it.Next(); obj != NULL; obj = it.Next()) {
     MarkBit mark_bit = Marking::MarkBitFrom(obj);
     ASSERT(Marking::IsWhite(mark_bit));
+    ASSERT_EQ(0, Page::FromAddress(obj->address())->LiveBytes());
   }
 }
 #endif
@@ -373,6 +374,7 @@ void MarkCompactCollector::ClearMarkbits() {
     MarkBit mark_bit = Marking::MarkBitFrom(obj);
     mark_bit.Clear();
     mark_bit.Next().Clear();
+    Page::FromAddress(obj->address())->ResetLiveBytes();
   }
 }
 
@@ -2592,12 +2594,10 @@ void MarkCompactCollector::ProcessWeakMaps() {
     ObjectHashTable* table = ObjectHashTable::cast(weak_map->table());
     for (int i = 0; i < table->Capacity(); i++) {
       if (MarkCompactCollector::IsMarked(HeapObject::cast(table->KeyAt(i)))) {
-        Object* value = table->get(table->EntryToValueIndex(i));
-        StaticMarkingVisitor::VisitPointer(heap(), &value);
-        table->set_unchecked(heap(),
-                             table->EntryToValueIndex(i),
-                             value,
-                             UPDATE_WRITE_BARRIER);
+        int idx = ObjectHashTable::EntryToValueIndex(i);
+        Object** slot =
+            HeapObject::RawField(table, FixedArray::OffsetOfElementAt(idx));
+        StaticMarkingVisitor::VisitPointer(heap(), slot);
       }
     }
     weak_map_obj = weak_map->next();