Oilpan: Avoid looking up TLS in ThreadState::current()

Source/platform/heap/ThreadState.cpp

Index: Source/platform/heap/ThreadState.cpp
diff --git a/Source/platform/heap/ThreadState.cpp b/Source/platform/heap/ThreadState.cpp
index b90d493bf63ac4bf71a69da752add1a52661eee5..2b9bf156b8b5244e879346bf7167833b095321ce 100644
--- a/Source/platform/heap/ThreadState.cpp
+++ b/Source/platform/heap/ThreadState.cpp
@@ -101,6 +101,7 @@ static void* getStackStart()
 static const size_t MaxPooledWrapperPersistentRegionCount = 2;
 
 WTF::ThreadSpecific<ThreadState*>* ThreadState::s_threadSpecific = 0;
+intptr_t ThreadState::s_mainThreadStackStart = 0;
 uint8_t ThreadState::s_mainThreadStateStorage[sizeof(ThreadState)];
 SafePointBarrier* ThreadState::s_safePointBarrier = 0;
 bool ThreadState::s_inGC = false;
@@ -311,6 +312,9 @@ ThreadState::ThreadState()
     ASSERT(!**s_threadSpecific);
     **s_threadSpecific = this;
 
+    if (isMainThread())
+        s_mainThreadStackStart = reinterpret_cast<intptr_t>(m_startOfStack);
+
     InitializeHeaps<NumberOfHeaps>::init(m_heaps, this);
 
     m_weakCallbackStack = new CallbackStack();