Because our ChromeClient always prohibits enterFullscreen for keyboard events. See http://trac.webkit.org/changeset/82124/trunk/Source/WebKit/gtk/WebCoreSupport/ChromeClientGtk.cpp
On mac the UIDelegate used by DRT always allows fullscreen, that's why the test passes there. We should do something similar on GTK, allow DRT to override this specific ChromeClient behavior.
Will skip the test on GTK for now.
A simpler solution would be to always allow enterFullscreen.
Seems okay now.