Oilpan: Introduce a state transition model for Oilpan GC states

Source/platform/heap/Heap.h

Index: Source/platform/heap/Heap.h
diff --git a/Source/platform/heap/Heap.h b/Source/platform/heap/Heap.h
index 903c0760dc81073a12d3d9704c4efbb378fb764f..3597c0018a1e567c92e767cc0f5ca68f041df314 100644
--- a/Source/platform/heap/Heap.h
+++ b/Source/platform/heap/Heap.h
@@ -919,7 +919,8 @@ public:
     static void globalWeakProcessing();
     static void setForcePreciseGCForTesting();
 
-    static void prepareForGC();
+    static void preGC();
+    static void postGC();
 
     // Conservatively checks whether an address is a pointer in any of the thread
     // heaps. If so marks the object pointed to as live.
@@ -945,6 +946,10 @@ public:
     // during conservative scanning.
     static bool lastGCWasConservative() { return s_lastGCWasConservative; }
 
+    static void enterGC() { ASSERT(!s_inGC); s_inGC = true; }
+    static void leaveGC() { ASSERT(s_inGC); s_inGC = false; }
+    static bool isInGC() { return s_inGC; }
+
     static FreePagePool* freePagePool() { return s_freePagePool; }
     static OrphanedPagePool* orphanedPagePool() { return s_orphanedPagePool; }
 
@@ -984,8 +989,8 @@ private:
         RegionTree* m_right;
     };
 
-    static void resetAllocatedObjectSize() { ASSERT(ThreadState::isAnyThreadInGC()); s_allocatedObjectSize = 0; }
-    static void resetMarkedObjectSize() { ASSERT(ThreadState::isAnyThreadInGC()); s_markedObjectSize = 0; }
+    static void resetAllocatedObjectSize() { ASSERT(Heap::isInGC()); s_allocatedObjectSize = 0; }
+    static void resetMarkedObjectSize() { ASSERT(Heap::isInGC()); s_markedObjectSize = 0; }
 
     static Visitor* s_markingVisitor;
     static CallbackStack* s_markingStack;
@@ -995,6 +1000,7 @@ private:
     static HeapDoesNotContainCache* s_heapDoesNotContainCache;
     static bool s_shutdownCalled;
     static bool s_lastGCWasConservative;
+    static bool s_inGC;
     static FreePagePool* s_freePagePool;
     static OrphanedPagePool* s_orphanedPagePool;
     static RegionTree* s_regionTree;