Implement aura isFullScreenMode() for desktop-X11 only

IsFullScreenMode() in fullscreen_aura was not implemented.
This CL provides an implementation of the IsFullScreenMode()
and is inspired by the GTK counterpart

Review URL: https://codereview.chromium.org/118703004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243289 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243289