Add xrandr output to chrome://system

This adds output of "xrandr --verbose" to chrome://system to
better troubleshoot monitor related issues.

BUG=chromium-os:26038
TEST=Tested on Lumpy. Verified that item "xrandr" shows in
     chrome://system and displays info when expanded.

Change-Id: I7be13e9800e7f3f6b850a58dd04f95a4fdedf3f4
Reviewed-on: https://gerrit.chromium.org/gerrit/16062
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Tested-by: Yufeng Shen <miletus@chromium.org>
Commit-Ready: Yufeng Shen <miletus@chromium.org>