Guard with X11 macro LinuxInputMethodContextFactory initialization

CL 25880002 implements InputMethodLinuxX11 and initializes it through
LinuxInputMethodContextFactory, which at the moment is used for X11 only. This
has broken Linux Ozone implementations that don't use X11. For now, we can
simply skip LinuxInputMethodContextFactory::SetInstance().

BUG=133476

Review URL: https://codereview.chromium.org/44453003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231308 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231308