Oilpan: Implement a GC to take a heap snapshot

Source/platform/heap/ThreadState.h

Index: Source/platform/heap/ThreadState.h
diff --git a/Source/platform/heap/ThreadState.h b/Source/platform/heap/ThreadState.h
index f8af346631fa121da15e5d4ba5d5858c4069318a..c8c6ed7cb780a82708b0cb19fe758058c3a0f7db 100644
--- a/Source/platform/heap/ThreadState.h
+++ b/Source/platform/heap/ThreadState.h
@@ -185,8 +185,14 @@ public:
     };
 
     enum GCType {
-        GCWithSweep, // Sweeping is completed in Heap::collectGarbage().
-        GCWithoutSweep, // Lazy sweeping is scheduled.
+        // Run a marking task and a sweeping task in Heap::collectGarbage().
+        GCWithSweep,
+        // Run only a marking task in Heap::collectGarbage(). A sweeping task
+        // is split into chunks and scheduled lazily.
+        GCWithoutSweep,
+        // Run a marking task just to take a heap snapshot. A sweeping task
+        // doesn't run and the marks are just dropped.
+        TakeSnapshot,
     };
 
     // See setGCState() for possible state transitions.
@@ -347,6 +353,10 @@ public:
     void preSweep();
     void completeSweep();
     void postSweep();
+    // makeConsistentForMutator() drops marks from marked objects and rebuild
+    // free lists. This is called after taking a snapshot and before resuming
+    // the executions of mutators.
+    void makeConsistentForMutator();
 
     // Support for disallowing allocation. Mainly used for sanity
     // checks asserts.
@@ -657,6 +667,7 @@ private:
     void unregisterPreFinalizerInternal(void*);
     void invokePreFinalizers(Visitor&);
 
+    void takeSnapshot();
 #if ENABLE(GC_PROFILING)
     void snapshotFreeList();
 #endif