linux aura: Fix handling focus change with enter/leave events.

It is possible for X11 to change the focus on enter/leave-notify events. Make
sure to handle such cases properly. This fixes keyboard input in a window when
there is no window manager present.

BUG=317856
R=erg@chromium.org

Review URL: https://codereview.chromium.org/72873002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235243 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235243