Linux Aura: Hide title-bar buttons when window is maximized on Unity.

BUG=322375
TEST=In Ubuntu Unity, maximize window. Close/minimize/maximize should be
in the global menu bar only, and not also in Chrome's tab strip.

Review URL: https://codereview.chromium.org/85003002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237866 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237866