Allow Oilpan heap objects account for their external allocations.

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 a2428428e52eb5fb9229509dee82db3436e9dfc5..ec8e1b1415c44809ee0f9d5817ee655704fcf474 100644
--- a/Source/platform/heap/ThreadState.h
+++ b/Source/platform/heap/ThreadState.h
@@ -343,6 +343,11 @@ public:
         return gcState() == Sweeping || gcState() == SweepingAndPreciseGCScheduled || gcState() == SweepingAndIdleGCScheduled;
     }
 
+    bool isSweepingScheduled() const
+    {
+        return gcState() == SweepingAndPreciseGCScheduled || gcState() == SweepingAndIdleGCScheduled;
+    }
+
     void preGC();
     void postGC(GCType);