[runtime] Introduce dedicated JSBoundFunction to represent bound functions.

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 4c5bd30aad881efdef1f75cfacd6ae2063bec28f..a2fd09e9f59d4edd27b4c6ca1c0b3800648e8729 100644
--- a/test/cctest/test-heap-profiler.cc
+++ b/test/cctest/test-heap-profiler.cc
@@ -251,7 +251,7 @@ TEST(BoundFunctionInSnapshot) {
       GetProperty(f, v8::HeapGraphEdge::kInternal, "bindings");
   CHECK(bindings);
   CHECK_EQ(v8::HeapGraphNode::kArray, bindings->GetType());
-  CHECK_EQ(3, bindings->GetChildrenCount());
+  CHECK_EQ(1, bindings->GetChildrenCount());
 
   const v8::HeapGraphNode* bound_this = GetProperty(
       f, v8::HeapGraphEdge::kShortcut, "bound_this");