Pipe the devTools FrameId from blink into the browser for headless

headless/public/headless_browser_context.h

Index: headless/public/headless_browser_context.h
diff --git a/headless/public/headless_browser_context.h b/headless/public/headless_browser_context.h
index 7f0b48ad0dcdf76c9d72386ddc89d99ba3d0c7f4..f94647a68f1cd75ccc0f0ccfd38b52cd12485cfb 100644
--- a/headless/public/headless_browser_context.h
+++ b/headless/public/headless_browser_context.h
@@ -59,6 +59,11 @@ class HEADLESS_EXPORT HeadlessBrowserContext {
   virtual HeadlessWebContents* GetWebContentsForDevToolsAgentHostId(
       const std::string& devtools_agent_host_id) = 0;
 
+  // Returns the frame tree node id for the corresponding RenderFrameHost or -1
+  // if it can't be found. Can be called on any thread.
+  virtual int GetFrameTreeNodeId(int render_process_id,
+                                 int render_frame_id) const = 0;
+
   // Destroy this BrowserContext and all WebContents associated with it.
   virtual void Close() = 0;