Fast path for ThreadSpecific for main thread on TLS-slow platforms

third_party/WebKit/Source/wtf/WTFThreadData.h

Index: third_party/WebKit/Source/wtf/WTFThreadData.h
diff --git a/third_party/WebKit/Source/wtf/WTFThreadData.h b/third_party/WebKit/Source/wtf/WTFThreadData.h
index a3351de1a4782adffc7051de70bf835f2becd445..a8228fce72e0e288153d3311b0ea25875cfe92b0 100644
--- a/third_party/WebKit/Source/wtf/WTFThreadData.h
+++ b/third_party/WebKit/Source/wtf/WTFThreadData.h
@@ -55,12 +55,20 @@ class WTF_EXPORT WTFThreadData {
 
   ThreadIdentifier threadId() const { return m_threadId; }
 
+#if OS(WIN) && COMPILER(MSVC)
+  static size_t threadStackSize();
+#endif
+
  private:
   std::unique_ptr<AtomicStringTable> m_atomicStringTable;
   std::unique_ptr<ICUConverterWrapper> m_cachedConverterICU;
 
   ThreadIdentifier m_threadId;
 
+#if OS(WIN) && COMPILER(MSVC)
+  size_t m_threadStackSize = 0u;
+#endif
+
   static ThreadSpecific<WTFThreadData>* staticData;
   friend WTFThreadData& wtfThreadData();
 };