Reapply r10158: Distinguish weak references in heap snapshots, group GC roots.

include/v8-profiler.h

Index: include/v8-profiler.h
diff --git a/include/v8-profiler.h b/include/v8-profiler.h
index f67646f54e77d90668e50491382777a46cd10d7b..27b3c6def3f5431d95e524f8a6cfbccc91f4524b 100644
--- a/include/v8-profiler.h
+++ b/include/v8-profiler.h
@@ -219,8 +219,9 @@ class V8EXPORT HeapGraphEdge {
                            // (e.g. parts of a ConsString).
     kHidden = 4,           // A link that is needed for proper sizes
                            // calculation, but may be hidden from user.
-    kShortcut = 5          // A link that must not be followed during
+    kShortcut = 5,         // A link that must not be followed during
                            // sizes calculation.
+    kWeak = 6              // A weak reference (ignored by the GC).
   };
 
   /** Returns edge type (see HeapGraphEdge::Type). */