Let Dart code running in Sky add events to the trace timeline

sky/engine/core/frame/LocalDOMWindow.cpp

Index: sky/engine/core/frame/LocalDOMWindow.cpp
diff --git a/sky/engine/core/frame/LocalDOMWindow.cpp b/sky/engine/core/frame/LocalDOMWindow.cpp
index 5d267952989b15efc908d6ce2fc76ef537cc2ad6..0342914477609820b17e2d41aad3e46793752f5b 100644
--- a/sky/engine/core/frame/LocalDOMWindow.cpp
+++ b/sky/engine/core/frame/LocalDOMWindow.cpp
@@ -57,6 +57,7 @@
 #include "sky/engine/core/frame/Location.h"
 #include "sky/engine/core/frame/Screen.h"
 #include "sky/engine/core/frame/Settings.h"
+#include "sky/engine/core/frame/Tracing.h"
 #include "sky/engine/core/inspector/ConsoleMessage.h"
 #include "sky/engine/core/loader/FrameLoaderClient.h"
 #include "sky/engine/core/page/ChromeClient.h"
@@ -418,6 +419,13 @@ Location& LocalDOMWindow::location() const
     return *m_location;
 }
 
+Tracing& LocalDOMWindow::tracing() const
+{
+    if (!m_tracing)
+        m_tracing = Tracing::create();
+    return *m_tracing;
+}
+
 DOMSelection* LocalDOMWindow::getSelection()
 {
     return m_frame->document()->getSelection();