Add is_mirrored_enabled() in ChameleonTest

to check if DUT is indeed in the mirrored mode after resuming from suspension
or reboot in mirrored mode display tests.

BUG=chromium:408376
TEST=manually run display_HotPlugAtSuspend.mirrored and
display_HotPlugAtBoot.mirrored on BIG and see the new error message being
reported.

Change-Id: I8c1f01d11f1f65b7b97e82933aaf48aaf02491c7
Reviewed-on: https://chromium-review.googlesource.com/222494
Reviewed-by: Wai-Hong Tam <waihong@chromium.org>
Tested-by: Hung-ying Tyan <tyanh@chromium.org>
Commit-Queue: Hung-ying Tyan <tyanh@chromium.org>
5 files changed