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

Source/core/dom/Fullscreen.cpp

Index: Source/core/dom/Fullscreen.cpp
diff --git a/Source/core/dom/Fullscreen.cpp b/Source/core/dom/Fullscreen.cpp
index 64b67a512ecf0eaffa02f3a1ab79bd769c43df31..69d008ebfa1c9e9ba7f85cdccef142fe476b5298 100644
--- a/Source/core/dom/Fullscreen.cpp
+++ b/Source/core/dom/Fullscreen.cpp
@@ -231,8 +231,13 @@ void Fullscreen::requestFullscreen(Element& element, RequestType requestType)
         //   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))