Fix ThreadChecker unittests when DCHECK_ALWAYS_ON is defined

This requires two fixes:
1. Use DCHECK instead of assert in ThreadChecker's unittest.

2. Activate DCHECK when DCHECK_ALWAYS_ON in enabled.

Both these modifications are in line with Chromium's implementation.
The ThreadChecker unittest was changed to use assert instead of DCHECK
on the initial import (since WebRTC did not have a DCHECK back then).

BUG=3803
TEST=local out/{Debug,Release}/rtc_unittests built with and without DCHECK_ALWAYS_ON
R=andrew@webrtc.org, pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/24569004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7178 4adac7df-926f-26a2-2b94-8c16560cd09d