New Heap Profiler: add API method for finding a graph node by id.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 19af866c24a80cbba08a3586326692e69dd3a71e..42c1db46194a2e5ac6060bf0d13521bb24c7fb8b 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -4872,6 +4872,13 @@ const HeapGraphNode* HeapSnapshot::GetRoot() const {
 }
 
 
+const HeapGraphNode* HeapSnapshot::GetNodeById(uint64_t id) const {
+  IsDeadCheck("v8::HeapSnapshot::GetNodeById");
+  return reinterpret_cast<const HeapGraphNode*>(
+      ToInternal(this)->GetEntryById(id));
+}
+
+
 const HeapSnapshotsDiff* HeapSnapshot::CompareWith(
     const HeapSnapshot* snapshot) const {
   IsDeadCheck("v8::HeapSnapshot::CompareWith");