Implement window management devtools commands for headless.

headless/lib/browser/headless_browser_impl.h

Index: headless/lib/browser/headless_browser_impl.h
diff --git a/headless/lib/browser/headless_browser_impl.h b/headless/lib/browser/headless_browser_impl.h
index 76acc0f4a5677de9571b4b1144d450a1afac3f57..485753de4d86d6cd17f909e145b0126699e897ed 100644
--- a/headless/lib/browser/headless_browser_impl.h
+++ b/headless/lib/browser/headless_browser_impl.h
@@ -72,6 +72,8 @@ class HEADLESS_EXPORT HeadlessBrowserImpl : public HeadlessBrowser,
   // (all web contents associated with it go away too).
   void DestroyBrowserContext(HeadlessBrowserContextImpl* browser_context);
 
+  HeadlessWebContentsImpl* GetWebContentsForWindowId(const int window_id);
+
   base::WeakPtr<HeadlessBrowserImpl> GetWeakPtr();
 
   // All the methods that begin with Platform need to be implemented by the