[DevTools] Use WebLayerTreeView::layerTreeId() for instrumentation.

Source/web/WebFrameWidgetImpl.h

Index: Source/web/WebFrameWidgetImpl.h
diff --git a/Source/web/WebFrameWidgetImpl.h b/Source/web/WebFrameWidgetImpl.h
index 5f7d03790108b67e60742b7b37e937913c38f1e0..2866b24078b2b6935783357d1824edeb1c45c1fa 100644
--- a/Source/web/WebFrameWidgetImpl.h
+++ b/Source/web/WebFrameWidgetImpl.h
@@ -140,6 +140,8 @@ public:
     // the page is shutting down, but will be valid at all other times.
     Page* page() const { return m_page; }
 
+    WebLayerTreeView* layerTreeView() const { return m_layerTreeView; }
+
     // Returns true if the event leads to scrolling.
     static bool mapKeyCodeForScroll(
         int keyCode,