chromeos: Silence log spam from x11_util.cc.
chromeos: Silence log spam from x11_util.cc.

The log spam happens when running browser_tests on Chrome OS, or running
Chrome for Chrome OS on Linux desktop.

The source of the spam was XGrabKey in
chrome/browser/chromeos/system_key_event_listener.cc.

We should use this class only when Chrome is running on Chrome OS in
non-test mode.

TEST=the log spam is gone from linux desktop, and function keys work on Chrome OS as before.
BUG=chromium-os:21331

Review URL: http://codereview.chromium.org/8380011

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@106994 0039d316-1c4b-4281-b951-d872f2087c98

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