Chameleon: Catch the failure of detecting internal screen

If no internal screen is reported from xrandr, a None value is returned
as the resolution. Should catch this failure.

BUG=None
TEST=Passed display_Resolution.mirrored

Change-Id: I420449e083ddfe1117f0fe39be51ac67f61d16af
Reviewed-on: https://chromium-review.googlesource.com/222910
Tested-by: Wai-Hong Tam <waihong@chromium.org>
Reviewed-by: Hung-ying Tyan <tyanh@chromium.org>
Commit-Queue: Wai-Hong Tam <waihong@chromium.org>
1 file changed