This is a list of Font Icons that can be used in Web Window/Dialogs for standard Window Controls such as Window Maximize/Restore, Minimize and Close. The examples below are standard Bootstrap Glyph Icons and Font Awesome Icons. While I may add other Window Control styles here, the ones listed below are all intended to mimic what Google uses for Inbox which is a beautifully modern Material Design User Interface.
Google Style Window/Dialog Controls
Expand to full-screen (Maximize)
fa-expand
http://fontawesome.io/icon/expand/
glyphicon glyphicon-resize-full
Exit full-screen (Restore)
fa-compress
http://fontawesome.io/icon/compress/
glyphicon glyphicon-resize-small
Minimize
fa-window-minimize
http://fontawesome.io/icon/window-minimize/
Close
fa-times or fa-close
http://fontawesome.io/icon/times/
glyphicon glyphicon-remove
Probably a better solution would just be to use a lowercase "x" as it isn't as thick as the Font Awesome or Glyphicons Icons.