[runtime] Use "the hole" instead of smi 0 as sentinel for context extension.

src/profiler/heap-snapshot-generator.cc

Index: src/profiler/heap-snapshot-generator.cc
diff --git a/src/profiler/heap-snapshot-generator.cc b/src/profiler/heap-snapshot-generator.cc
index 49057ed0bc2c040de3afc27f18f866fb80a8b4aa..5d1a711af67beaeed1cdf4a4107b4ff0702744b5 100644
--- a/src/profiler/heap-snapshot-generator.cc
+++ b/src/profiler/heap-snapshot-generator.cc
@@ -1247,7 +1247,7 @@ void V8HeapExplorer::ExtractContextReferences(int entry, Context* context) {
   }
   EXTRACT_CONTEXT_FIELD(CLOSURE_INDEX, JSFunction, closure);
   EXTRACT_CONTEXT_FIELD(PREVIOUS_INDEX, Context, previous);
-  EXTRACT_CONTEXT_FIELD(EXTENSION_INDEX, Object, extension);
+  EXTRACT_CONTEXT_FIELD(EXTENSION_INDEX, HeapObject, extension);
   EXTRACT_CONTEXT_FIELD(NATIVE_CONTEXT_INDEX, Context, native_context);
   if (context->IsNativeContext()) {
     TagObject(context->normalized_map_cache(), "(context norm. map cache)");