Introduce JavascriptExecutionObserver scope.

src/execution.cc

Index: src/execution.cc
diff --git a/src/execution.cc b/src/execution.cc
index cc067a8ab88ca61ca95878f68d96eb49fd5f815d..01005977e588a135ff4b893846f5ca3ec4ed9f9c 100644
--- a/src/execution.cc
+++ b/src/execution.cc
@@ -68,6 +68,7 @@ MUST_USE_RESULT MaybeHandle<Object> Invoke(Isolate* isolate, bool is_construct,
     isolate->ReportPendingMessages();
     return MaybeHandle<Object>();
   }
+  JavascriptExecutionObserver::SetObserved(isolate);
 
   // Placeholder for return value.
   Object* value = NULL;
@@ -535,5 +536,27 @@ Object* StackGuard::HandleInterrupts() {
   return isolate_->heap()->undefined_value();
 }
 
+
+JavascriptExecutionObserver::JavascriptExecutionObserver(Isolate* isolate)
+    : isolate_(isolate), observed_(false) {
+  prev_ = isolate->execution_observer();
+  isolate->set_execution_observer(this);
+}
+
+
+JavascriptExecutionObserver::~JavascriptExecutionObserver() {
+  isolate_->set_execution_observer(prev_);
+}
+
+
+void JavascriptExecutionObserver::SetObserved(Isolate* isolate) {
+  for (JavascriptExecutionObserver* observer = isolate->execution_observer();
+       observer != NULL; observer = observer->prev_) {
+    // If this observer is already set to true, so are all its predecessors.
+    if (observer->observed_) break;
+    observer->observed_ = true;
+  }
+}
+
 }  // namespace internal
 }  // namespace v8