Add WeakCell support to heap profiler.

test/cctest/test-heap-profiler.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 674 matching lines @@
   const v8::HeapGraphNode* z =
       GetProperty(global, v8::HeapGraphEdge::kProperty, "z");
   CHECK(z);
   const v8::HeapGraphNode* map =
       GetProperty(z, v8::HeapGraphEdge::kInternal, "map");
   CHECK(map);
   CHECK(GetProperty(map, v8::HeapGraphEdge::kInternal, "map"));
   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) {
   v8::Isolate* isolate = CcTest::isolate();
   v8::HandleScope scope(isolate);
   v8::Local<v8::ObjectTemplate> global_template =
       v8::ObjectTemplate::New(isolate);
   global_template->SetInternalFieldCount(2);
   LocalContext env(NULL, global_template);
   v8::Local<v8::Object> global_proxy = env->Global();
@@ skipping 2377 matching lines @@
       "      a[i] = i;\n"
       "  for (var i = 0; i < 3; ++i)\n"
       "      a.shift();\n"
       "}\n");
 
   CcTest::heap()->CollectGarbage(v8::internal::NEW_SPACE);
   // Should not crash.
 
   heap_profiler->StopSamplingHeapProfiler();
 }