x11: Select for touch-events on the X root window only on chromeos.

Selecting for touch-events on the X11 root window causes an unexplained
XInputExtension error on some environments on linux desktop (e.g. the error
happens with unity, but not with icewm or a number of other window managers).

BUG=302696
R=derat@chromium.org

Review URL: https://codereview.chromium.org/99333009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238972 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238972