Add a UseCounter for legacy fullscreen key event filtering

Source/core/page/EventHandler.cpp

Index: Source/core/page/EventHandler.cpp
diff --git a/Source/core/page/EventHandler.cpp b/Source/core/page/EventHandler.cpp
index 72fa0d6c79d7d4cfd16fee07b47385aaf4b85b98..37ef13a3e2a087661d23232add50db7a536788e8 100644
--- a/Source/core/page/EventHandler.cpp
+++ b/Source/core/page/EventHandler.cpp
@@ -2946,8 +2946,10 @@ bool EventHandler::keyEvent(const PlatformKeyboardEvent& initialKeyEvent)
 
     ASSERT(m_frame->document());
     if (FullscreenElementStack* fullscreen = FullscreenElementStack::fromIfExists(*m_frame->document())) {
-        if (fullscreen->webkitIsFullScreen() && !isKeyEventAllowedInFullScreen(fullscreen, initialKeyEvent))
+        if (fullscreen->webkitIsFullScreen() && !isKeyEventAllowedInFullScreen(fullscreen, initialKeyEvent)) {
+            UseCounter::count(*m_frame->document(), UseCounter::KeyEventNotAllowedInFullScreen);
             return false;
+        }
     }
 
     if (initialKeyEvent.windowsVirtualKeyCode() == VK_CAPITAL)