Notify in console when an API fails because it needs to be invoked on user action.

Source/core/dom/Fullscreen.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
@@ skipping 213 matching lines @@
         // node document:
 
         // The fullscreen element ready check returns false.
         if (!fullscreenElementReady(element, requestType))
             break;
 
         // This algorithm is not allowed to show a pop-up:
         //   An algorithm is allowed to show a pop-up if, in the task in which the algorithm is running, either:
         //   - an activation behavior is currently being processed whose click event was trusted, or
         //   - the event listener for a trusted click event is being handled.
-        if (!UserGestureIndicator::processingUserGesture())
+        if (!UserGestureIndicator::processingUserGesture()) {
+            String message = ExceptionMessages::failedToExecute("requestFullScreen",
+                "Element", "API can only be initiated by a user gesture.");
+            document()->executionContext()->addConsoleMessage(
+                ConsoleMessage::create(JSMessageSource, WarningMessageLevel, message));
             break;
+        }
 
         // Fullscreen is not supported.
         if (!fullscreenIsSupported(element.document(), element))
             break;
 
         // 2. Let doc be element's node document. (i.e. "this")
         Document* currentDoc = document();
 
         // 3. Let docs be all doc's ancestor browsing context's documents (if any) and doc.
         Deque<Document*> docs;
@@ skipping 357 matching lines @@
 void Fullscreen::trace(Visitor* visitor)
 {
     visitor->trace(m_fullScreenElement);
     visitor->trace(m_fullScreenElementStack);
     visitor->trace(m_fullScreenRenderer);
     visitor->trace(m_eventQueue);
     DocumentSupplement::trace(visitor);
 }
 
 } // namespace blink