linux: Fix touch-event handling when the touch-screen is an attached slave.

Touchscreen device on chromeos is always a floating slave device. Touch works best
on linux if the touchscreen is a floating slave device too. But most other linux
apps don't process that well, and the touchscreen is usually an attached slave
device on these systems. So make sure chrome does receive touch-events correctly
from such devices.

BUG=327224
TBR=erg@chromium.org

Review URL: https://codereview.chromium.org/120503002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242363 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242363