Tidy DEFINE_(THREAD_SAFE_)STATIC_LOCAL() implementations.

third_party/WebKit/Source/bindings/core/v8/ScriptWrappable.h

Index: third_party/WebKit/Source/bindings/core/v8/ScriptWrappable.h
diff --git a/third_party/WebKit/Source/bindings/core/v8/ScriptWrappable.h b/third_party/WebKit/Source/bindings/core/v8/ScriptWrappable.h
index 86f7ae5098299f762aa7592a496f247be059e03b..ac6b70148178b05bc00aec0c84a1077a686e3c84 100644
--- a/third_party/WebKit/Source/bindings/core/v8/ScriptWrappable.h
+++ b/third_party/WebKit/Source/bindings/core/v8/ScriptWrappable.h
@@ -61,7 +61,14 @@ class CORE_EXPORT ScriptWrappable : public TraceWrapperBase {
   WTF_MAKE_NONCOPYABLE(ScriptWrappable);
 
  public:
-  ScriptWrappable() {}
+  ScriptWrappable() {
+#if DCHECK_IS_ON()
+    // If this DCHECK() triggers, you're attempting to embed a ScriptWrappable
+    // inside a static local singleton, which is unsafe.
+    // See |DEFINE_STATIC_LOCAL()| documentation for further details.
+    DCHECK(ThreadState::current()->canAllocateScriptWrappable());
+#endif
+  }
 
   bool isScriptWrappable() const override { return true; }