Oilpan: simplify away StoppingOtherThreads pseudo GCState.

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 3b2c8d649be43b4dc04a4e88083f4b4c0947efe3..31c03102bc0bd9fd1b81d2dde95aa1c4b87c20f1 100644
--- a/Source/platform/heap/ThreadState.h
+++ b/Source/platform/heap/ThreadState.h
@@ -188,7 +188,6 @@ public:
         IdleGCScheduled,
         PreciseGCScheduled,
         FullGCScheduled,
-        StoppingOtherThreads,
         GCRunning,
         EagerSweepScheduled,
         LazySweepScheduled,
@@ -348,7 +347,11 @@ public:
     void leaveNoAllocationScope() { m_noAllocationCount--; }
     bool isGCForbidden() const { return m_gcForbiddenCount; }
     void enterGCForbiddenScope() { m_gcForbiddenCount++; }
-    void leaveGCForbiddenScope() { m_gcForbiddenCount--; }
+    void leaveGCForbiddenScope()
+    {
+        ASSERT(m_gcForbiddenCount > 0);
+        m_gcForbiddenCount--;
+    }
     bool sweepForbidden() const { return m_sweepForbidden; }
 
     void prepareRegionTree();
@@ -555,7 +558,6 @@ public:
     }
     void leaveGCForbiddenScopeIfNeeded(GarbageCollectedMixinConstructorMarker* gcMixinMarker)
     {
-        ASSERT(m_gcForbiddenCount > 0);
         if (m_gcMixinMarker == gcMixinMarker) {
             leaveGCForbiddenScope();
             m_gcMixinMarker = nullptr;