Oilpan: Implement a GC to take a heap snapshot

Source/platform/heap/Visitor.h

 /*
  * Copyright (C) 2013 Google Inc. 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
@@ skipping 310 matching lines @@
 //
 // Pointers within objects are traced by calling the |trace| methods
 // with the object as an argument. Tracing objects will mark all of the
 // contained pointers and push them on the marking stack.
 class PLATFORM_EXPORT Visitor : public VisitorHelper<Visitor> {
 public:
     friend class VisitorHelper<Visitor>;
     friend class InlinedGlobalMarkingVisitor;
 
     enum MarkingMode {
+        // This is a default visitor. This is used for GCType=GCWithSweep
+        // and GCType=GCWithoutSweep.
         GlobalMarking,
+        // This visitor does not trace objects outside the heap of the
+        // GCing thread. This is used for GCType=ThreadTerminationGC.
         ThreadLocalMarking,
+        // This visitor just marks objects and ignores weak processing.
+        // This is used for GCType=TakeSnapshot.
+        SnapshotMarking,
+        // This visitor is used to trace objects during weak processing.
+        // This visitor is allowed to trace only already marked objects.
         WeakProcessing,
     };
 
     virtual ~Visitor() { }
 
     using VisitorHelper<Visitor>::mark;
 
     // This method marks an object and adds it to the set of objects
     // that should have their trace method called. Since not all
     // objects have vtables we have to have the callback as an
@@ skipping 79 matching lines @@
     {
         DEFINE_STATIC_LOCAL(String, typenameString, (WTF::extractTypeNameFromFunctionName(WTF::extractNameFunction<T>())));
         return typenameString;
     }
 };
 #endif
 
 } // namespace blink
 
 #endif // Visitor_h