Add WeakCell support to heap profiler.

test/cctest/test-heap-profiler.cc

Index: test/cctest/test-heap-profiler.cc
diff --git a/test/cctest/test-heap-profiler.cc b/test/cctest/test-heap-profiler.cc
index 8e91e4742fdc258c2e565f99c810aa3b2f4cfc77..637bda144b4b6666de0f322b083f1d02dd03318e 100644
--- a/test/cctest/test-heap-profiler.cc
+++ b/test/cctest/test-heap-profiler.cc
@@ -692,7 +692,9 @@ TEST(HeapSnapshotMap) {
   CHECK(GetProperty(map, v8::HeapGraphEdge::kInternal, "prototype"));
   CHECK(GetProperty(map, v8::HeapGraphEdge::kInternal, "back_pointer"));
   CHECK(GetProperty(map, v8::HeapGraphEdge::kInternal, "descriptors"));
-  CHECK(GetProperty(map, v8::HeapGraphEdge::kInternal, "weak_cell_cache"));
+  const v8::HeapGraphNode* weak_cell =
+      GetProperty(map, v8::HeapGraphEdge::kInternal, "weak_cell_cache");
+  CHECK(GetProperty(weak_cell, v8::HeapGraphEdge::kWeak, "value"));
 }
 
 TEST(HeapSnapshotInternalReferences) {