Introduce JavascriptExecutionObserver scope.

src/execution.h

Index: src/execution.h
diff --git a/src/execution.h b/src/execution.h
index f1b1057a61439bcdc40eb51df425fdbc9770acc4..faf984ca583357e91322e87cf03cc1adb6699516 100644
--- a/src/execution.h
+++ b/src/execution.h
@@ -247,6 +247,24 @@ class StackGuard final {
   DISALLOW_COPY_AND_ASSIGN(StackGuard);
 };
 
+
+// Stack-allocated object to track Javascript execution within its scope.
+class JavascriptExecutionObserver BASE_EMBEDDED {
+ public:
+  explicit JavascriptExecutionObserver(Isolate* isolate);
+  ~JavascriptExecutionObserver();
+
+  static void SetObserved(Isolate* isolate);
+  bool observed() const { return observed_; }
+
+ private:
+  Isolate* isolate_;
+  JavascriptExecutionObserver* prev_;
+  bool observed_;
+
+  DISALLOW_COPY_AND_ASSIGN(JavascriptExecutionObserver);
+};
+
 }  // namespace internal
 }  // namespace v8