Oilpan: handle thread-local weak tracing slightly better.

Source/platform/heap/Heap.cpp

Index: Source/platform/heap/Heap.cpp
diff --git a/Source/platform/heap/Heap.cpp b/Source/platform/heap/Heap.cpp
index b5a83c52067a3b29b2328cbd3d8cae6bcb49f3b6..31bf0fe1f7aefa2f65a75283f10b080acda2a9ed 100644
--- a/Source/platform/heap/Heap.cpp
+++ b/Source/platform/heap/Heap.cpp
@@ -2297,8 +2297,8 @@ void Heap::collectGarbage(ThreadState::StackState stackState, ThreadState::GCTyp
     ThreadState::NoAllocationScope noAllocationScope(state);
 
     preGC();
-    StackFrameDepth::configureStackLimit();
-    ASSERT(StackFrameDepth::isSafeToRecurse());
+
+    StackFrameDepthScope stackDepthScope;
 
     size_t totalObjectSize = Heap::allocatedObjectSize() + Heap::markedObjectSize();
     Heap::resetHeapCounters();
@@ -2358,8 +2358,7 @@ void Heap::collectGarbageForTerminatingThread(ThreadState* state)
         ThreadState::NoAllocationScope noAllocationScope(state);
 
         state->preGC();
-        StackFrameDepth::configureStackLimit();
-        ASSERT(StackFrameDepth::isSafeToRecurse());
+        StackFrameDepthScope stackDepthScope;
 
         // 1. Trace the thread local persistent roots. For thread local GCs we
         // don't trace the stack (ie. no conservative scanning) since this is